[Sidebars endpoint] Save function-based widgets

[adamziel]

Description

As @jorgefilipecosta noticed in https://github.com/WordPress/gutenberg/pull/24290/files#r468740440:

Right now the non-class based widgets seem to not be saved.
I tested with this simple widget https://themes.trac.wordpress.org/browser/silesia/1.0.6/include/widgets/feedburner.php
(pasted it on functions.php)
And I verified the values on the form never get saved.

The root cause is the API endpoint getting confused about widget IDs like feedburner-1 vs feedburner. Function-based widgets may be registered following either pattern and the API needs to account for that. This PR solves this problem.

How has this been tested?

1. Add the feedburner widget mentioned above to your local WP.
2. Go to the experimental widget management screen.
3. Add a legacy widget and set it to Feedburner.
4. Update title and feedburner ID.
5. Click save.
6. Refresh the page.
7. Confirm the widget is rendered with the settings you provided earlier.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: 0 B

Total Size: 1.16 MB

ℹ️ View Unchanged

Filename	Size	Change
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build/annotations/index.js	3.67 kB	0 B
build/api-fetch/index.js	3.44 kB	0 B
build/autop/index.js	2.82 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	7.97 kB	0 B
build/block-directory/style-rtl.css	953 B	0 B
build/block-directory/style.css	952 B	0 B
build/block-editor/index.js	126 kB	0 B
build/block-editor/style-rtl.css	10.7 kB	0 B
build/block-editor/style.css	10.7 kB	0 B
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build/block-library/editor.css	8.5 kB	0 B
build/block-library/index.js	133 kB	0 B
build/block-library/style-rtl.css	7.42 kB	0 B
build/block-library/style.css	7.43 kB	0 B
build/block-library/theme-rtl.css	729 B	0 B
build/block-library/theme.css	730 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	47.7 kB	0 B
build/components/index.js	200 kB	0 B
build/components/style-rtl.css	15.7 kB	0 B
build/components/style.css	15.7 kB	0 B
build/compose/index.js	9.67 kB	0 B
build/core-data/index.js	12.3 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.55 kB	0 B
build/date/index.js	5.38 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	4.47 kB	0 B
build/edit-navigation/index.js	11.6 kB	0 B
build/edit-navigation/style-rtl.css	1.16 kB	0 B
build/edit-navigation/style.css	1.16 kB	0 B
build/edit-post/index.js	304 kB	0 B
build/edit-post/style-rtl.css	5.61 kB	0 B
build/edit-post/style.css	5.61 kB	0 B
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build/edit-site/style-rtl.css	3.06 kB	0 B
build/edit-site/style.css	3.06 kB	0 B
build/edit-widgets/index.js	11.7 kB	0 B
build/edit-widgets/style-rtl.css	2.45 kB	0 B
build/edit-widgets/style.css	2.45 kB	0 B
build/editor/editor-styles-rtl.css	537 B	0 B
build/editor/editor-styles.css	539 B	0 B
build/editor/index.js	45.3 kB	0 B
build/editor/style-rtl.css	3.8 kB	0 B
build/editor/style.css	3.79 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.71 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	711 B	0 B
build/keyboard-shortcuts/index.js	2.52 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.12 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.32 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.41 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/rich-text/index.js	13.9 kB	0 B
build/server-side-render/index.js	2.77 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.85 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

_{compressed-size-action}

[draganescu]

Solved the problem :)