Allow LSP server to use builtin watcher on Windows and macOS (#4288)

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 3 people

internal/fswatch/watcher.go

@@ -48,6 +48,11 @@ type Watcher interface {
 	Name() string
 	// Available reports whether this watcher works on the current OS.
 	Available() bool
+	// HasFastRecursiveBackend reports whether this watcher supports efficient
+	// recursive watching without requiring a full userspace tree walk. This is
+	// true for Windows (ReadDirectoryChangesW subtree mode) and macOS FSEvents
+	// (inherently recursive), and false for all other backends.
+	HasFastRecursiveBackend() bool
 	// WatchDirectory watches dir for changes, calling fn with batched
 	// events. By default, only direct children are watched. Use
 	// [WithRecursive] to watch the entire directory tree.
@@ -214,6 +219,16 @@ func (w *watcher) String() string  { return w.name }
 func (w *watcher) Available() bool { return w.factory != nil }
 func (w *watcher) unexported()     {}
 
+// HasFastRecursiveBackend implements [Watcher.HasFastRecursiveBackend].
+func (w *watcher) HasFastRecursiveBackend() bool {
+	switch w.name {
+	case "windows", "fsevents":
+		return true
+	default:
+		return false
+	}
+}
+
 func (w *watcher) getImpl() (watcherImpl, error) {
 	w.mu.Lock()
 	if w.impl != nil {