Improve plugin-view toolbar lifecycle

[colin-grant-work]

What it does

Fixes #9075.
Closes #9080.
This PR builds on #9080. That PR addressed a bug in the fetching of values from workspace scope in the MonacoConfigurationService, but did not fix the problem with missing events for updating toolbars in plugin-contributed views. This PR makes several changes to address the latter problem:

• Removes the .with utility from the ViewContextKeyService. That utility was designed to check whether a when expression's conditions would be satisfied given certain assumptions, but it caused two or four change events to be fired by the ContextKeyService for every check. It has been replaced by a .with utility on the MonacoContextKeyService class that uses Monaco utilities to create subcontexts so that the events don't bubble to the global context.
• Adds a local context to plugin view widgets so that tabbar toolbar checks no longer need to use the .with utility at all.
• Separates the ContextKeyService interface from the mostly useless dummy implementation in core. This is a breaking change (extenders can no longer extends ContextKeyService), but it allows us to document the functionality of the methods. Previously each method was just annotated with Should be implemented elsewhere.

How to test

• Add the Maven plugin.
• Click on the flat / hierarchical item.
• Observe that when you click, it changes to the other item consistently.
• Check that other tabbar toolbar items and context-dependent items are working as expected.

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]

I can confirm that the issue exists on master and is addressed by this change.

• The file tree/list icon correctly changes using the maven extension
• Other buttons remain working as expected (no regressions)

The code looks good to me 👍