feat(zeitwerk): Add types for v2.8

[tobidelius]

Adds RBS for zeitwerk, which doesn't ship its own sig/ directory.
Signatures are written against zeitwerk 2.8.3 and cover the public, documented API:

• Zeitwerk::Loader class methods: for_gem, for_gem_extension, eager_load_all, eager_load_namespace, all_dirs, default_logger
• Zeitwerk::Loader configuration: push_dir, dirs, tag, inflector, logger, ignore, collapse, do_not_eager_load, nsfile, enable_reloading
• Callbacks: on_setup, on_load, on_unload
• Lifecycle and eager loading: setup, reload, eager_load, eager_load_dir, eager_load_namespace, load_file
• Introspection: all_expected_cpaths, cpath_expected_at
• Inflector, GemInflector, NullInflector, and the error classes

APIs that zeitwerk marks as undocumented or experimental (unload, unregister, unloadable_cpaths, Zeitwerk.with_loader) and internals are intentionally left out.

Notes for reviewers

• Loader#inflector returns Zeitwerk::Inflector, not the _Inflector interface, so that the documented loader.inflector.inflect(...) idiom type-checks without a cast. The setter accepts any _Inflector. This is a small trade-off in soundness when a custom inflector is assigned.
• The value yielded to on_load/on_unload callbacks is untyped, since it's whatever class or module the file defines.

I've been using these types in my own project, so they're somewhat battle-tested.

[github-actions]

@tobidelius Thanks for your contribution!

Please follow the instructions below for each change.
See also: https://github.com/ruby/gem_rbs_collection/blob/main/docs/CONTRIBUTING.md

Available commands

You can use the following commands by commenting on this PR.

• /merge: Merge this PR if CI passes

---

zeitwerk

This RBS files are newly added.

You can merge this PR immediately if the CI passes.
Just comment /merge to merge this PR.

[tobidelius]

/merge