WIP sub nav fixes for navigation block

[karmatosed]

This has a little nicer dropdown, however, isn't complete and a work in progress for now. This will fix #18310 once done.

To do:

• Mobile styling.
• Make arrow indicator change styles.
• Fix bottom margin issue.
• Test with multiple sub menus.
• Iterate design to have less spacing and no text underline.

To test:

• Multiple themes.
• Mobile devices.

Note: the idea is to have a drop-down with no JS so keeping it simple.

Once that is done then code review.

[mtias]

Most of these styles should likely be added within the theme.scss styles, which is what gets loaded when a theme does add_theme_support( 'wp-block-styles' ). Particularly the ones that are harder to replace, like ::before.

[karmatosed]

I am going to close this in favour of #18350 as can then work off it.