add ghcXX-minimal-ghc-web flavor: wasm + JS backend dev tooling

The wasm and JavaScript backends need a small set of dev-time tools
that aren't part of a stock Haskell shell:

  * nodejs_22   — required for utils/jsffi/post-link.mjs (uses
                  import.meta.filename added in Node 20.11; Ubuntu's
                  apt nodejs is 18.x and silently breaks the post-link
                  step) and for JSFFI host execution at test time
  * wabt        — wasm-objdump for inspecting custom sections (e.g.
                  detecting ghc_wasm_jsffi imports) in wasm modules
  * wasmtime    — pure-WASI runtime, when a wasm module has no JSFFI
                  imports and can run standalone
  * emscripten  — the JavaScript backend's C toolchain (emcc / em++ /
                  emar / emnm / emranlib / emstrip)

Currently downstream consumers (e.g. stable-haskell/ghc's wasm
cross-compiler CI) bootstrap these per-platform via apt + NodeSource +
nix-env + curl installers, plus PATH workarounds for devx scrubbing
/usr/bin. Shipping them in the flavor collapses ~70 lines of
platform-shell to one `shell:` line in user workflows.

Deliberately NOT bundled: wasi-sdk. Its version needs to match the
wasm32-wasi-ghc cross-compiler bundle that ghc-wasm-meta
(https://gitlab.haskell.org/ghc/ghc-wasm-meta) owns, so keeping that
pin in ghc-wasm-meta avoids version drift across two trees. Users
still bootstrap wasi-sdk via ghc-wasm-meta for wasm32-wasi builds.

Closure cost on aarch64-darwin (paired with the prior trim commit):

  ghc98-minimal-ghc      (current master) :  6.12 GB
  ghc98-minimal-ghc      (after trim PR)  :  4.00 GB  (-2.12 GB)
  ghc98-minimal-ghc-web  (this commit)    :  5.99 GB  (+1.99 GB)

Net: the web flavor with the full toolchain ends up SMALLER than the
current untrimmed minimal-ghc, because the trim commit removed
ghc-9.10.3 and emscripten's LLVM/apple-sdk now dedupes against the
shell's base nixpkgs pin (no version fragmentation).

Comfortably under the 10 GB GitHub Actions per-repo cache cap on
both Darwin and Linux (Linux delta is similar magnitude — emscripten
+ closure-compiler are the heavy hitters on both).

Verified inside the patched shell:

  $ ghc --version            # 9.8.4
  $ cabal --version          # 3.17.0.0
  $ happy --version          # 2.1.7
  $ alex --version           # 3.5.4.0
  $ git --version            # 2.51.2 (gitMinimal)
  $ node --version           # v22.21.1
  $ wasm-objdump --version   # 1.0.37
  $ wasmtime --version       # 38.0.3
  $ emcc --version           # 4.0.12-git

All on the expected store paths.

Author: angerman

dynamic.nix

@@ -1,5 +1,5 @@
 # define a development shell for dynamically linked applications (default)
-{ self, pkgs, compiler, compiler-nix-name, toolsModule, withHLS ? true, withHlint ? true, withIOG ? true, withIOGFull ? false, withGHCTooling ? false }:
+{ self, pkgs, compiler, compiler-nix-name, toolsModule, withHLS ? true, withHlint ? true, withIOG ? true, withIOGFull ? false, withGHCTooling ? false, withWebTooling ? false }:
 let iog = import ./iog-libs.nix { inherit pkgs; };
     tool-version-map = (import ./tool-map.nix) self;
     tool = tool-name: pkgs.pkgsBuildBuild.haskell-nix.tool compiler-nix-name tool-name [(tool-version-map compiler-nix-name tool-name) toolsModule];
@@ -78,6 +78,7 @@ pkgs.mkShell {
                    + lib.optionalString withIOG                  "-iog"
                    + lib.optionalString withIOGFull              "-full"
                    + lib.optionalString withGHCTooling           "-ghc"
+                   + lib.optionalString withWebTooling           "-web"
                    ;
         in ''
         export PS1="\[\033[01;33m\][\w]$\[\033[00m\] "
@@ -154,6 +155,27 @@ pkgs.mkShell {
           libffi.dev
         ]
       )
+      ++ optionals withWebTooling (
+        # Tooling for the wasm and JavaScript backends. wasi-sdk itself is
+        # deliberately not bundled — its version needs to match the
+        # wasm32-wasi-ghc cross-compiler bundle, which ghc-wasm-meta owns
+        # (https://gitlab.haskell.org/ghc/ghc-wasm-meta). Users still bootstrap
+        # wasi-sdk via ghc-wasm-meta's installer for wasm32-wasi cross-builds.
+        # What we DO bundle here:
+        #   * nodejs_22  — required for post-link.mjs (uses import.meta.filename
+        #                  added in Node 20.11; Ubuntu's apt nodejs is 18.x)
+        #                  and for JSFFI host execution
+        #   * wabt       — wasm-objdump for inspecting wasm modules
+        #   * wasmtime   — pure-WASI runtime (when no JSFFI imports)
+        #   * emscripten — the JavaScript backend's C toolchain (emcc / em++
+        #                  / emar / emnm / emranlib / emstrip)
+        with pkgs; [
+          nodejs_22
+          wabt
+          wasmtime
+          emscripten
+        ]
+      )
     ;
 
     passthru = {

flake.nix

@@ -225,6 +225,10 @@
                   pkgs.lib.nameValuePair "${short-name}-minimal-ghc" (
                     import ./dynamic.nix (args // { withHLS = false; withHlint = false; withIOG = false; withGHCTooling = true; })
                   )) (compilers pkgs)
+              // pkgs.lib.mapAttrs' (short-name: args:
+                  pkgs.lib.nameValuePair "${short-name}-minimal-ghc-web" (
+                    import ./dynamic.nix (args // { withHLS = false; withHlint = false; withIOG = false; withGHCTooling = true; withWebTooling = true; })
+                  )) (compilers pkgs)
               // pkgs.lib.mapAttrs' (short-name: args:
                   pkgs.lib.nameValuePair "${short-name}-static" (
                     import ./static.nix (args // { withIOG = false; })