Fix AMP violations caused by AMP dev mode regression

[felixarntz]

Bug Description

#505 introduced support for AMP dev mode so that Site Kit scripts and stylesheets are not considered AMP violations by the plugin, since they are only loaded when the user is logged in anyway and should continue to work in these circumstances.

There was a regression recently (reported multiple times) by no longer using wp_add_inline_script() to print the window.googlesitekit object definition and instead relying on WP_Scripts::add_data() (because this is the only way to make it work as a standalone script). Since this script is standalone, we need to add back a mechanism to highlight it as compatible with AMP dev mode. We can use a /*googlesitekit*/ comment for that like before (unfortunately modifying the rendered script tag is not possible).

Another issue is that the AMP plugin filter for that, amp_dev_mode_element_xpaths, fires rather early, but we only hook it in during wp_enqueue_scripts which is too late. It can be generally hooked in during init, there is nothing expensive in there anyway.

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Acceptance criteria

• There should not be any AMP violations flagged by the AMP plugin when the Site Kit admin bar menu is loaded.

Implementation Brief

• Introduce Assets::add_extra_script_amp_dev_mode() to add /*googlesitekit*/ comment to all 'data' scripts.
• Hook in amp_dev_mode_element_xpaths in top-level Admin_Bar::register().
• Include hoverintent-js fix so that it is flagged with data-ampdevmode. This was added to the AMP plugin via Include admin-bar script deps in dev mode (e.g. hoverintent-js) ampproject/amp-wp#3928, but only in the very recent 1.4.2, so I think it's worth doing that here, with a TODO to remove in the future.
• Merge Fix AMP dev mode support #1143

QA Brief

• Activate AMP plugin and set to AMP standard mode.
• Go to a page in the frontend for which the Site Kit admin bar menu shows.
• Ensure the plugin does not report any validation errors, and that the menu opens as expected when hovering over it.

Changelog entry

• Fix AMP violations when user is logged in and Site Kit admin bar menu is active.

[aaemnnosttv]

IB + CR ✅

[aaemnnosttv]

✅ QA

Using AMP v1.4.0 and Site Kit 1.3.0

1. Confirmed validation errors on home page
   
2. Hover intent and Site Kit global data
   
3. ✅ Rechecking shows no validation errors using Site Kit 3b532c9
   

Using AMP v1.4.4 (current latest) and Site Kit 1.3.0

1. Confirmed validation errors on home page
   
2. Site Kit global data only
   
3. ✅ Rechecking shows no validation errors using Site Kit 3b532c9
   

✅ Menu opens as expected on hover

Moving to approval