Handle activeCustomEditorId (#13219)

[dhuebner]

What it does

Tracks activeCustomEditorId context key next to existing activeWebviewPanelId

How to test

It is a bit tricky to test it manually and I found no test spec to re-use to test context keys.

• To test it manually checkout custom editor sample.
• Change the catCustoms.pawDraw.new command contribution by adding "enablement": "activeCustomEditorId == 'catCustoms.catScratch'"
• build the extension using vsce package. Note: You will need to change the publisher in the package json to something meaningful.
• install cat-customs-0.0.1.vsix into Theia
• create a file named test.cscratch and open it
• A command CMD+Shift+P Create new Paw Draw Document should only be visible when the cscratch custom editor is active

Review checklist

• [x ] 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

[rschnekenbu]

That looks good to me!

[msujew]

Looks good to me as well 👍