Improve editor preference dendrification

[colin-grant-work]

What it does

Fixes #11709 by improving the dendrification of preferences - the old code would clobber e.g. editor.scrollbar.scrollByPage when it encountered editor.scrollbar.horizontal - and passing user values for editor.scrollbar.* preferences, along with defaults for internal (not user-accessible) values.

How to test

1. Set the preference editor.scrollbar.scrollByPage to true. (and set editor.find.addExtraSpaceOnTop to false and editor.hover.delay to something long)
2. Open a big file (I like common-frontend-contribution.ts).
3. Click on the scrollbar some distance from its current position.
4. Observe that your preference is respected.
5. Check the other two preference as well.

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 preferences now work as expected with the steps to test 👍