[meta] Remove dependency on Underscore.js outside of Backbone and mark it internal

Retitling for clarity.

I can help you with this one!

So sorry about my delay here, this is my first time deprecating a JS lib, I made this new patch for D9.4, deprecating it from the core.libraries, do we need deprecation tests here?

Regarding the proposed solution:

1. Deprecate Underscore.js library, The patch adds a new deprecation message for this.
2. Load Underscore.js directly on libraries that currently depend on it, I don't think I got this right, the patch adds a new underscore lib on each module that depends on it and points to the file directly on core, I didn't know if it was supposed to be this way.
3. Open follow-up issues to remove all usages of Underscore.js, We have 4 modules using underscore, editor, filter, image and quickedit

Since we still need to open the follow-ups issues and review the patch. I'll leave this as NW.

Just a note that quickedit is going to be removed in 10.0.0 in #3227033: Remove Quick Edit from core. The follow-up for that should be posted in the contrib module, I think.

Can we even pass tests if core itself is using a deprecated asset library? If not, we'd need to do the removal in one issue or block the deprecation on the child issues?

Usually we have to do the issues to remove the usage first, and then do the final issue to remove the deprecation. We could convert this issue to a meta issue to do that?

QuickEdit will remain in Drupal 9 core until 2024, so many issues would need to be fixed in both core and contrib QuickEdit until then. In the case of this, though, we could maybe skip/silence the deprecation within QuickEdit since it's essentially end-of-life. Worth exploring, anyway.

We could convert this issue to a meta issue to do that?

Sure! IMHO it's the best idea here, then we could open the follow up issues for the modules: editor, filter, image and quickedit

I thought about this more.

QuickEdit will need to retain its dependency on underscore regardless, because it depends on Backbone. So that's not worth doing; we can just silence the deprecation within QuickEdit. I wonder if it's possible to ignore the deprecation messages only within QuickEdit...

Regarding #10, QuickEdit's tests will all be marked legacy when the module is formally deprecated, so yes! That will solve that; just means QE deprecation needs to be complete first.

Scoping by module isn't always the best choice, but after taking a quick look at the library definitions, we have the following JS code affected

drupal.editor.admin:
  version: VERSION
  js:
    js/editor.admin.js: {}
  dependencies:
    - core/jquery
    - core/once
    - core/jquery.once.bc
    - core/drupal
    - core/underscore

drupal.filter.filter_html.admin:
  version: VERSION
  js:
    filter.filter_html.admin.js: {}
  dependencies:
    - core/jquery
    - core/once
    - core/jquery.once.bc
    - core/underscore

quickedit.inPlaceEditor.image:
  version: VERSION
  js:
    js/editors/image.js: {}
    js/theme.js: {}
  css:
    component:
      css/editors/image.css: {}
    theme:
      css/editors/image.theme.css: {}
  dependencies:
    - core/jquery
    - core/drupal
    - core/underscore
    - quickedit/quickedit

So Image is only dependent on editor in its QuickEdit integration, which is code that should have been moved to QuickEdit already. 🤔 We should figure out why that hasn't happened yet from the QuickEdit deprecation meta.

Since it's really only two JS files, core/modules/editor/js/editor.admin.js and core/modules/filter/filter.filter_html.admin.js, we should be able to handle the removals in a single issue. That issue can go in anytime. Then, we commit the actual deprecation once QuickEdit is deprecated.

Updated the IS with a new proposed resolution. We need two child issues filed.

Filed the first child issue for the removal from non-deprecatable code.

Added #3272872: deprecate underscore and mark it internal to handle the backbone dependency on underscore.

Is there anything left in scope here for D11?

This is done, we have #3203920: Replace Contextual Links BackboneJS usage with VanillaJS equivalent one remaining child issue (apart from deprecating toolbar when navigation is stable) to remove backbone too.