feat!: add shortcuts to navigate between headings in the flyout

[maribethb]

The basics

• I validated my changes

The details

Resolves

Fixes #9618

Proposed Changes

• Adds H and Shift+H shortcuts to jump between headings in the flyout. shortcut has no effect if the flyout is not focused.
• Pops a toast if you press "enter" while on a flyout header (not a button) telling you to find a block or use a different key to switch headings

Reason for Changes

Screenreader users found it confusing to navigate flyouts and kept trying to "insert" while focused on a heading

Test Coverage

Added (ai-assisted) tests and manually tested

BREAKING CHANGE

This is only breaking because it adds two new keyboard shortcuts for H and shift+H. If you don't already have shortcuts registered for these keys, it does not break you.
if you do have shortcuts registered for those keys, you may be able to keep both of them if your shortcut does not trigger in the flyout.
If the shortcuts do collide, we recommend that you change the keybinding for one of them so that screenreader users still have this navigational aid.