vfs: dispatch fs/promises to mounted VFS instances

Add mount/unmount lifecycle on `VirtualFileSystem`, a handler registry
that fs.js and fs/promises.js consult via `vfsState.handlers`, and a
router that maps absolute paths to the VFS that owns them. When a VFS
is mounted, the public `fs.*` and `fs/promises` APIs (including
streams, `fs.watch`, and `opendir`) dispatch to the provider for paths
under the mount point, and fall through to the real filesystem
otherwise. Includes per-method dispatch tests, error-path coverage,
multi-mount routing tests, and router unit tests.
Ref: #63115
Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: #63537
Refs: #63115
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>

Author: mcollina

lib/fs.js

@@ -31,6 +31,7 @@ const {
  getDirent,
  getOptions,
  getValidatedPath,
+ vfsState,
 }=require('internal/fs/utils');
 const{
  validateFunction,
@@ -330,6 +331,20 @@ function opendir(path, options, callback) {
 callback=typeofoptions==='function' ? options : callback;
 validateFunction(callback,'callback');
 
+consth=vfsState.handlers;
+if(h!==null){
+try{
+constresult=h.opendirSync(path,options);
+if(result!==undefined){
+process.nextTick(callback,null,result);
+return;
+}
+}catch(err){
+process.nextTick(callback,err);
+return;
+}
+}
+
 path=getValidatedPath(path);
 options=getOptions(options,{
 encoding: 'utf8',
@@ -354,6 +369,12 @@ function opendir(path, options, callback) {
 }
 
 functionopendirSync(path,options){
+consth=vfsState.handlers;
+if(h!==null){
+constresult=h.opendirSync(path,options);
+if(result!==undefined)returnresult;
+}
+
 path=getValidatedPath(path);
 options=getOptions(options,{encoding: 'utf8'});