List v2: add __experimentalEnableListBlockV2 flag for mobile

[ellatrix]

What?

Adds a flag for mobile to know if the site supports List v2.
See #40089, but for List v2 this time.
See #42624 for request.

Why?

Mobile needs to know when to load the v1 and the v2 of the list block.

How?

By adding a __experimentalEnableQuoteBlockV2 flag to the block editor settings endpoint.

Testing Instructions

• Comment lines 22 and 23 here so the code runs for any request, and not only for a 'mobile' request.
• Go to "Gutenberg > Experiments" and enable the list v2 experiment.
• Load the post editor, open the devtools, and execute wp.apiFetch({path: '/wp-block-editor/v1/settings/', method: 'GET'}). The expected result is that the flag __experimentalEnableListBlockV2 is present and is true.
• Go to "Gutenberg > Experiments" and disable the list v2 experiment.
• Load the post editor, open the devtools, and execute wp.apiFetch({path: '/wp-block-editor/v1/settings/', method: 'GET'}). The expected result is that the flag __experimentalEnableListBlockV2 is present and is false.

Screenshots or screencast

[geriux]

LGTM! I've tested this and it's returning the flag correctly. Thank you for adding this 🎉