Handle Language Overrides in setPreference

[colin-grant-work]

What it does

Fixes #10663 by checking whether the preference name passed to setPreference contains a language override identifier and handling that case appropriately.

How to test

1. Check out this branch, which is this branch with one additional commit for demonstration purposes.
2. Open Theia with the Theia repo as your workspace.
3. Open a TS file.
4. Open the workspace preferences (.theia/settings.json), if you'd like.
5. Run the command Overrides: Set language overrides
6. You should end up with this in your settings.json

"[typescript]": {
    "editor.tabSize": 3,
    "editor.fontSize": 18
}

and those settings should take effect in the TS file.
7. Run the command Overrides: Remove tabsize override
8. It should remove "editor.tabSize" from the [typescript] object in the settings.json, and you should get the default tabSize in your TS file.
9. Run the command Overrides: Remove Typescript overrides.
10. It should delete the [typescript] section entirely, and you should get default behavior for both preferences.
11. If the tests in this PR pass, it shows basically the same thing as those steps do manually.

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]

The changes look good to me!

• Running the Set language overrides command changes the settings.json file as expected and the changes are instantly applied to the editor
• Resetting each of the preferences apply the change correctly to the settings.json and editor