Description

This PR replaces the navigation block submenu triangle with a chevron. Before:

After:

Whether the triangle or the chevron is a subject of taste, hwoever the chevron brings with it a few themer benefits. Specifically, it is an SVG with two strokes and a stroke-width. Which means a theme can:

• Change the stroke-width thicker or thinner
• Remove the stroke-width and replace it with a fill, restoring it to a triangle

In that vein, the chevron SVG shape is to an extent more flexible.

How has this been tested?

Please test navigation blocks with custom submenus, and with page list, and on the navigation screen. Verify the chevron looks right.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.