Reload GDExtension when editor is already focused

### Describe the project you are working on

GDExtension with C++

### Describe the problem or limitation you are having in your project

When building GDExtension via IDE and waiting until build is finished, then switching to editor, we can see the changes that were made to the extension. However, if we start build and switch to the editor before it completes, upon build completion we will not see the changes until the editor is refocused (lose focus and gain focus again).

### Describe the feature / enhancement and how it helps to overcome the problem or limitation

It helps see the most recent changes made to the extension without having to refocus the editor.

### Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Ideally, editor should be listening to file changes and reload extensions whenever is detects changes.

### If this enhancement will not be used often, can it be worked around with a few lines of script?

Don't think so.

### Is there a reason why this should be core and not an add-on in the asset library?

This is core functionality.

No files, tests, or entry points are named. First reproduce the C++ GDExtension build while the Godot editor remains focused, then trace how the editor currently notices changes after focus is regained. Done means a completed build reloads the extension and shows its changes without requiring the editor to lose and regain focus.

Written by the indexing model from the issue text.