Improve autoposition (container element boundaries)

Hi, first of all thanks for the simple and lean component!

Would you consider improving on the autoposition feature, so that you can optionally specify a containing element?

I'd like to use the tooltip in a modal and the viewport boundaries aren't very helpful for auto positioning.
Here's a REPL: https://svelte.dev/repl/d2afb26070f54cf8af1bc5a7265d814e?version=4.2.0
