[monaco] Fix bad preference initialization

[svenefftinge]

first register override identifier, because monaco will immediately
update already opened documents and then initialize with
bad preferences.

Fixes #6449

What it does

The plug-in initialization happens slightly after the editors have been created.
When a new language is registered, monaco will update the options of matching editors immediately.
But our preferenceService doesn't know yet about the override identifier and returns a bad preference model (undefined values).

This change registers the override identifier first.

How to test

• Run Theia with built-in VS Code extensions.
• Open an editor and make sure minimaps are disabled.
• now reload and see if the editor has a minimap (not with this fix).

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[akosyakov]

It works nicely again, thank you!