Edit Widget: Add Basic plugin support

[t-hamano]

Discussed in #41034

Related: #34460

We suggest adding basic plugin support to the widget editor as well, to make it more consistent with the post editor and the site editor. Please advise us of any issues that need to be resolved to make this happen.

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^{Originally posted by davidwebca May 13, 2022}
Hi everyone,

I have a small plugin that allows to edit the width of the sidebar in the block editor (https://github.com/davidwebca/wordpress-menu-classes). I currently prevent the enqueueing of the JS code in the new block editor in widgets because it used to crash. Now I realize it doesn't crash, but it doesn't work either. Am I missing something? The 5.8 announcement clearly states that "The Widgets screen is extendable via block editor APIs such as registerPlugin [...]" but it doesn't seem to be the case at all.

Maybe I'm doing something wrong, maybe I'm importing the JS modules from the wrong source, etc. but I can't seem to find any documentation on this specific issue. Any help would be welcome!

Thanks a lot ✌️

[rmorse]

Seconding this, I would like to see consistency across the site editor, post editor, and (classic) widget editor - currently the widget editor is the only one where this is not possible.