feat: Add pyrefly completion command

[pavelzw]

Summary

Fixes #4882

Test Plan

$ cargo run -- completion --shell fish > pyrefly.fish
$ cargo run -- completion --shell bash > pyrefly.bash
$ cargo run -- completion --shell zsh > pyrefly.zsh

# fish
$ source pyrefly.fish
$ pyrefly <TAB>
bazel-check                                                 (Entry point for Bazel integration)
buck-check                                                   (Entry point for Buck integration)
check                                               (Full type checking on a file or a project)
completion                                       (Generate a shell completion script on stdout)
coverage                                                               (Type coverage commands)
dump-config  (Dump info about pyrefly's configuration. Use by replacing `check` with `dump-co…)
help                                (Print this message or the help of the given subcommand(s))
infer                               (Automatically add type annotations to a file or directory)
init  (Initialize a new pyrefly config in the given directory, or migrate an existing mypy or…)
lsp                                                                       (Start an LSP server)
report  (Deprecated alias for `pyrefly coverage report`. Use `pyrefly coverage report` instead)
snippet                                                           (Check a Python code snippet)
stubgen                                     (Generate .pyi stub files from Python source files)
suppress             (Suppress type errors by adding ignore comments, or remove unused ignores)
tsp                                                                        (Start a TSP server)

# bash
$ source pyrefly.bash
$ pyrefly <TAB>
-j           --threads    --version    dump-config  lsp          report
-v           --color      check        buck-check   tsp          suppress
-h           --verbose    completion   bazel-check  infer        stubgen
-V           --help       snippet      init         coverage     help

# zsh
$ source pyrefly.zsh
$ pyrefly <TAB>
bazel-check  -- Entry point for Bazel integration
buck-check   -- Entry point for Buck integration
check        -- Full type checking on a file or a project
completion   -- Generate a shell completion script on stdout
coverage     -- Type coverage commands
dump-config  -- Dump info about pyrefly's configuration. Use by replacing `check` with `dum
help         -- Print this message or the help of the given subcommand(s)
infer        -- Automatically add type annotations to a file or directory
init         -- Initialize a new pyrefly config in the given directory, or migrate an exist
lsp          -- Start an LSP server
report       -- Deprecated alias for `pyrefly coverage report`. Use `pyrefly coverage repor
snippet      -- Check a Python code snippet
stubgen      -- Generate .pyi stub files from Python source files
suppress     -- Suppress type errors by adding ignore comments, or remove unused ignores
tsp          -- Start a TSP server

this can then get added to the homebrew formula like in https://github.com/Homebrew/homebrew-core/blob/fbd5caa399af11f0e4bb74c014204bbefc9318a6/Formula/p/pixi.rb#L45 or to the conda recipe like in https://github.com/conda-forge/ty-feedstock/blob/14216677727cdb373eb51ff871a225cec2f10cea/recipe/recipe.yaml#L51 to enable shell completions for all brew / pixi users automatically

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D119480372. (Because this pull request was imported automatically, there will not be any future comments.)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅