Fix EuiToolTip text reflow on content changes

[thompsongl]

Summary

Fixes #1734, in which content changes near the right edge of the window caused longer text to wrap lines and result in incorrect, uncorrected positioning calculations.

• Uses EuiResizeObserver instead of EuiMutationObserver. The latter did not account for side effect updates and missed possible resize changes. (This is a more forward-looking change, getting at what I think we actually want to observe).
• Calculates EuiToolTip x-axis position from the right if nearing the right edge of the window. This prevents text wrap/reflow, negating positioning correction.

Checklist

- [ ] This was checked in mobile

• This was checked in IE11

- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs
- [ ] Documentation examples were added

• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately

- [ ] Jest tests were updated or added to match the most common scenarios
- [ ] This was checked against keyboard-only and screenreader scenarios
- [ ] This required updates to Framer X components

[thompsongl]

For testing you can load up TSVB in Kibana as mentioned in the issue. Otherwise I had success simulating in EUI docs by manually positioning a button.

Gif of Proof (in Kibana):

[chandlerprall]

LGTM; pulled and tested the case with some modified doc examples