fix(fs): walk does not apply exts filter to directories

[LeSingh1]

Fixes #6736.

walk and walkSync ran the exts filter against the directory name in include() when deciding whether to yield a directory entry. Directories generally don't have file extensions, so any exts option effectively dropped every directory from the output, even with includeDirs:true (the default).

Repro before the patch:

import { walkSync } from "jsr:@std/fs/walk";
for (const e of walkSync("./project", { includeDirs: true, exts: ["ts"] })) {
  console.log(e.path, e.isDirectory ? "(dir)" : "(file)");
}
// only "*.ts" files - every directory is missing

After:

./project (dir)
./project/sub (dir)
./project/sub/b.ts (file)
./project/a.ts (file)

The fix passes undefined for exts in the includeDirs branch of both walk and walkSync. The match and skip regex filters still apply to directories. The !followSymlinks symlink emission branch is unchanged because a symlink's name still has an extension just like the target.

Tests:

• Existing walk() / walkSync() accepts ext option as strings tests updated to include the root directory in the expected entries (the default includeDirs:true now actually yields it).
• Two new tests pin the includeDirs:false case so the intent stays explicit.

All ext-related tests pass. The four pre-existing * regExps failures in fs/walk_test.ts reproduce on origin/main (verified by stashing the patch and re-running) so they're unrelated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (9fc18f1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7167   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files         636      636           
  Lines       52142    52142           
  Branches     9401     9401           
=======================================
  Hits        49315    49315           
  Misses       2249     2249           
  Partials      578      578           

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