Demonstrates wiring gleam_gazelle (this repo's Gazelle language extension, see
gleam_gazelle) into a downstream project's own gazelle_binary, as
opposed to examples/smoke etc., which hand-write their gleam_package(...) targets directly.
BUILD.bazel here is checked in exactly as bazel run //:gazelle would generate it from
gleam.toml + src/**/*.gleam + test/**/*.gleam -- if you add a new .gleam file, re-run
bazel run //:gazelle to keep it in sync (it won't happen automatically).