Dispose of child instantiation services in MonacoEditor

[colin-grant-work]

What it does

Fixes #15242 by including a child instantiation service created as part of editor creation in the disposables for that editor.

How to test

1. Put breakpoints on instantiation service creation / disposal, or modify the code in node_modules/@theia/monaco-editor-core/esm/vs/platform/instantiation/common/instantiationService.js to provide logs.
2. Open an editor.
3. Four instantiation services should be created.
4. Close the editor.
5. Four instantiation services should be disposed of.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

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

[sdirix]

I debugged the instantiation as suggested. I can confirm that createChild is called 4 times per instantiation and was only disposed 2 times. With the PR I have the same number of instantiations and disposals. Good job!