fix(fs): expose iterator helpers on walk/walkSync and expandGlob/expandGlobSync

[fibibot]

Summary

walk, walkSync, expandGlob, and expandGlobSync are generator functions, but their return-type annotations widened the type to (Async)IterableIterator<WalkEntry>, hiding the ES2025 iterator helpers (.map, .filter, .take, …) at the type level. The runtime objects do have those helpers (on the sync side), so the original repro

const a = walkSync(".");
const b = a.map((v) => v.name);
//          ^^^ Property 'map' does not exist on type 'IterableIterator<WalkEntry>'.

was a type-only bug.

This PR changes the annotations to Generator<WalkEntry> /
AsyncGenerator<WalkEntry> for all four functions so callers can chain
helpers without widening, and the async pair is forward-compatible for
when async iterator helpers ship in the standard library.

Note: walk and walkSync need an explicit return type (TS7023 — they
reference themselves recursively), so the annotations are tightened
rather than dropped entirely.

Test plan

Regression tests are co-located in fs/walk_test.ts and
fs/expand_glob_test.ts. The sync tests exercise .map(...).toArray()
at both type-check and runtime; the async tests assert that
AsyncGenerator shape is preserved (no async iterator helpers exist at
runtime yet, so they just exercise .next() / .return()).

• deno fmt --check
• deno lint
• deno run -A _tools/check_docs.ts
• New tests pass: walkSync()/expandGlobSync() returns a generator that exposes iterator helpers, walk()/expandGlob() returns an async generator
• Original repro from fs: walkSync returns a type without iterator helpers #7099 type-checks

Closes #7099
Closes bartlomieju/orchid-inbox#62

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (6510612) to head (384a722).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7136   +/-   ##
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  Coverage   94.61%   94.61%           
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  Files         634      634           
  Lines       51822    51826    +4     
  Branches     9336     9348   +12     
=======================================
+ Hits        49029    49035    +6     
+ Misses       2218     2216    -2     
  Partials      575      575           

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