Fix extra file watcher events causing unnecessary Node.js restarts

[JeremyTCD]

We use the framework FileSystemWatcher type to watch for file changes and restart Node.js when they occur. FileSystemWatcher is known to fire extra events on windows. To work around this, we were stopping FileSystemWatcher when we received an event:

Javascript.NodeJS/src/NodeJS/NodeJSServiceImplementations/OutOfProcess/OutOfProcessNodeJSService.cs

Line 593 in 3f5eb53

_fileWatcher?.Stop();

and restarting it only after moving to a new Node.js process:

Javascript.NodeJS/src/NodeJS/NodeJSServiceImplementations/OutOfProcess/OutOfProcessNodeJSService.cs

Line 461 in 3f5eb53

_fileWatcher?.Start();

After 144, moving to a new process sped up to the point where we would sometimes:

1. Receive a file changed event
2. Stop the watcher
3. Move to a new process
4. Start the watcher
5. Receive an extra of the initial file changed event
6. Move to a new process again, unnecessarily

Unnecessary Node.js restarts caused tests to fail (e.g. first failure has 7 "file changed" logs when there should only be 5).

This PR fixes the issue by debouncing file changed events.