vfs: dispatch fs/promises to mounted VFS instances

[mcollina]

• Add mount/unmount on VirtualFileSystem plus a handler registry consulted by lib/fs.js, lib/internal/fs/promises.js, and lib/internal/fs/dir.js via vfsState.handlers.
• Route absolute paths to the VFS that owns them, so the public fs.* and fs/promises APIs (sync, callback, promise, streams, fs.watch, opendir) dispatch to the mounted provider and fall through to the real filesystem otherwise.

Ref: #63115

[codecov]

Codecov Report

❌ Patch coverage is 92.69521% with 116 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.35%. Comparing base (74ccf38) to head (2e86633).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/vfs/setup.js	86.64%	87 Missing and 1 partial ⚠️
lib/internal/fs/promises.js	93.29%	10 Missing and 1 partial ⚠️
lib/internal/vfs/file_system.js	93.93%	8 Missing ⚠️
lib/fs.js	99.26%	4 Missing ⚠️
lib/internal/fs/dir.js	80.95%	4 Missing ⚠️
lib/internal/vfs/router.js	97.82%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63537      +/-   ##
==========================================
+ Coverage   90.32%   90.35%   +0.03%     
==========================================
  Files         730      732       +2     
  Lines      234209   235705    +1496     
  Branches    43934    44443     +509     
==========================================
+ Hits       211558   212982    +1424     
- Misses      14372    14452      +80     
+ Partials     8279     8271       -8     

Files with missing lines	Coverage Δ
lib/internal/fs/utils.js	98.44% <100.00%> (+<0.01%)	⬆️
lib/internal/vfs/errors.js	100.00% <100.00%> (ø)
lib/internal/vfs/router.js	97.82% <97.82%> (ø)
lib/fs.js	98.33% <99.26%> (+0.13%)	⬆️
lib/internal/fs/dir.js	93.84% <80.95%> (-0.74%)	⬇️
lib/internal/vfs/file_system.js	99.21% <93.93%> (+2.52%)	⬆️
lib/internal/fs/promises.js	92.94% <93.29%> (+0.06%)	⬆️
lib/internal/vfs/setup.js	86.64% <86.64%> (ø)

... and 46 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73672/