Navigation: Allow multiple navigations with the same ref

[scruffian]

What?

Our last fix for preventing recursive navigation blocks (#46387) prevents users from having the same navigation twice in one page. This allows that use case while still preventing nested navigation blocks.

Fixes #47085 (comment)

Why?

Users should be allowed to create multiple navigation menus on the same page, with the same ref.

How?

Use a recursive loop to check whether a block or any inner blocks contain a navigation block.

Testing Instructions

1. Try adding two navigation menus with the same ref to a post/template. You should see both of them.
2. You can also try testing nested navigation menus by adding <!-- /wp:navigation --> to a wp_navigation CPT, but you'll need to use mysql. (See Navigation Block: Prevent circular references in navigation block rendering #46387 for instructions).

Testing Instructions for Keyboard

As above.

Screenshots or screencast

Two identical navigations on the same page:

[draganescu]

Code LGTM - there's some linting. I totally support this for now. If we ever figure we need navigation blocks inside navigation blocks we can then check for ref equality as well.

[getdave]

Nice work. Please can we reference the bug report Issue in this PR description so we're clear what this PR is fixing?

[getdave]

Suggested change

	function block_core_navigation_has_nested_core_navigation( $parsed_blocks ) {
	function block_core_navigation_block_contains_core_navigation( $parsed_blocks ) {

Nit: I think we can just name this to explain that it's testing a set of blocks for the presence of a core navigation block. The argument is "a collection of blocks" not "a core navigation block".

It's not "does this core navigation block contain core navigation blocks".

[github-actions]

Flaky tests detected in 575db6b.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4014912047
📝 Reported issues:

• [Flaky Test] should always expand single line selection #39787 in specs/editor/various/multi-block-selection.test.js