Optimize BP Nouveau for Twenty Twenty-four integration

[imath]

As we are used to make sure BuddyPress is optimized for each new WordPress default theme. Let's respect this tradition for 12.0.0.

I believe we should also avoid displaying the customizer when a Block Theme is in use and simplify the layout for these themes.

That's why I'm compensating by introducing a Priority nav extending the Twenty Nineteen Priority Menu for our purpose. This Priority nav is used in Single item pages.

Optimize BP Nouveau for Twenty Twenty-four

The above screenshot shows the BP Priority Nav theme compat feature.

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/9030

[imath]

In 13652:

Introduce a new Theme Compat feature firstly designed for Block Themes

NB : this feature is also only available in the BP Nouveau template pack.

The Priority Navigations theme compat feature is used to make sure BP Single items primary and secondary navigations are displayed on a single row/line no matter how many items these navigations are containing.
When there's not enough space to display all the items on this row/line, an ellipsis is inserted and hovering on it shows the remaining nav items into a dropdown menu.

This commit also make sure the BP Customizer sections are neutralized so that the Customizer is not loaded on front-end when a Block Theme is active to start complying with the fact these kind of themes are not using the Customizer.

See #9030
See ​https://github.com/buddypress/buddypress/pull/197

[imath]

In 13653:

BP Nouveau: add a Twenty Twenty-Four companion stylesheet

This new companion stylesheet improves the way BP Nouveau integrates with the latest WordPress default theme (the one bundled with version 6.4).

Fixes #9030
Closes ​https://github.com/buddypress/buddypress/pull/197