A VS Code extension configurationDefault does not work #12713

[jfaltermeier]

What it does

Fixes: #12713

• plugin contribution handler will now also register configurationDefault for non language related configuration changes
• extend preference-contribution#doSetSchema to allow modifying default values for existing schemas when contributed via a special schema id
• also update select options in preference-select-input, since the default value may change now, which requires the SelectOptions to recreate

The solution is based on the existing implementation for providing language specific editor settings:
#3540
#4296
In the existing solution we are using a special defaultOverrides PreferenceSchema that gets registered. I've adapted the registration code to also work for existing preference keys when provided via this special schema.

I am open for suggestions to implement this in a different way, however we should probably refactor the language specific editor overrides then as well.

How to test

See the bug report for a reproducer: #12713
Check the preference's (default) value when installing/uninstalling the extension.
Check the preference UI (default highlight in drop down) and test resetting to the default value

Please be aware of this bug during testing with the vsix from the reproducer: #10812

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

[msujew]

Looks pretty good to me. I've used this extension for testing, which changes the editor.fontSize configuration for all editors to 20 and for markdown editors to 30. 👍

• The defaults are correctly applied in the preferences.
• Changing to the original default value highlights the preference as changed and resetting it changes it back to the config default override.
• Deinstalling the plugin and reloading the app correctly resets the preference defaults.
• Changing the preferences without installing a plugin that changes the defaults still works as expected.

I have a minor suggestion, see below.