Use ResizeObserver for monaco editor automaticLayout

[nrayburn-tech]

Replaces the MutationObserver with ResizeObserver. Uses a timeout for any browser that does not support ResizeObserver. (Safari is the only one of the currently supported browsers without ResizeObserver.)
When resizing an editor with a dynamic size (i.e. a percentage for the width), the editor should resize automatically.

This PR fixes #1855, fixes #1878 in monaco-editor.
Might fix #1884 as well.

This PR replaces #90111.

[nrayburn-tech]

I am not sure if the types for ResizeObserver belong at the top of the file, they are temporary because they are not currently in typescript, ts issue.

The setInterval() function defaults to a type of NodeJS.Timeout, so it is being cast to unknown and then number. I don't know if this is really the correct way to handle this.

I dropped the private _onDidMutate method to use the public observe method instead since they were both doing the same thing. I didn't think both methods needed to be kept.

@alexdima let me know if anything needs to be changed, I just wanted to mention a few concerns before marking it as "Ready for review".

[alexdima]

This is great, thank you!