Add flex resize, focus mode, hard-coded limits for now (clone of #2704)

[sander777]

This feature had been implemented in https://github.com/helix-editor/helix/pull/2704, but PR seams inactive. I really wanted this feature and just copy pasted code from original PR.

Fixes #1176

[the-mikedavis]

If you're re-submitting someone else's code it's customary to credit them as a co-author in the commits: https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors

[archseer]

If it's as far as simply rebasing the original work I'd keep the commit authorship as is and either add a Co-authored-by or a separate commit with the fixes

[sander777]

There was a problem with author's name on old commits. Now everything looks fine

[EzekielEnns]

some day....