Godot DAP "Completions Request" from the DAP spec is not implemented

### Tested versions

Reproducible on master after https://github.com/godotengine/godot/pull/97585 is merged, because the ability to evaluate expressions in debugger was introduced.

### System information

All OS

### Issue description

"Completions Request" from the DAP [spec](https://microsoft.github.io/debug-adapter-protocol//specification.html) is not implemented

### Steps to reproduce

Start debugging with DAP in vscode.
Stop on a brakepoint
In the Watch area try to type `self.` and there would be no completion.

### Minimal reproduction project (MRP)

Empty project with a scene and one gd script is enough.

Start with the DAP specification's Completions Request and the existing expression-evaluation path introduced by PR 97585. Trace how debugger requests reach the Watch area, then verify that typing `self.` while stopped at a breakpoint produces completions in VS Code.

Written by the indexing model from the issue text.