[preferences] Reject invalid preference schemas.

[AlexTugarev]

What it does

Fixes #6106

How to test

1. install rust vscode extension (0.6.1) (includes an invalid pref schema) and e.g. the python vscode extension
2. start the Theia application and see the rejection message for the invalid rust schema is logged, but more importantly it now should be possible to change python preferences (e.g. python.pythonPath) which was effectively a no-op before.

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

[AlexTugarev]

@akosyakov if you (or anyone else) is interested, I'm happy to change this to throw an error so it can be handled e.g. in the plugin system in order to make it easier to identity the causing part.

In this version the rejection message will contain only the pref key e.g. rust-client.channel, which should help to identify the contribution.

[akosyakov]

@AlexTugarev throwing is a breaking change, so maybe better no

[AlexTugarev]

because of the download issues for debug adapters, one need to disable them. here is a prepared workspace as a snapshot.

[akosyakov]

@AlexTugarev please file an issue about debug adapters, we need to find a solution for it

[svenefftinge]

@AlexTugarev please file an issue about debug adapters, we need to find a solution for it

Maybe we should get rid of the debug-nodejs extension completely?

[akosyakov]

There are more extensions doing it as well as docker images.

[AlexTugarev]

here #6111

[akosyakov]

Tested that invalid schema does not break the combined preference schema and schema validation errors are still logged. Thanks @AlexTugarev!