Restore selection in Monaco editor options

[colin-grant-work]

What it does

Fixes #11017 by restoring the logic to extract the selection from the options passed to the MonacoEditorService. Previously, the relevant fields had been added to the IResourceEditorInput declaration in our monaco.d.ts, but in fact they come from an extension of that interface, the ITextResourceEditorInput. The new code checks whether the input we're dealing with satisfies the latter and extracts the selection if possible.

How to test

1. ctrl+click on variables / methods imported from / defined in other files.
2. Observe that the file opens and scrolls to the definition / declaration of the relevant method.

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

[vince-fugnitto]

I confirm that the changes fix the issue 👍