Dynamic title attribute for markdown footnotes

[edent]

Fixes #27387

Quick conformance test:

Lorem ipsum dolor sit amet[^latin]. These footnotes support Markdown[^md] and HTML[^html]. They don't choke on entities[^ent]! And quote marks are fine[^q]. What about very long footnotes[^long]? Fully multilingual[^zh] and Unicode[^🥰] compatible.

[^latin]: Other Latin texts are available
[^md]: Some Markdown *emphasis* and a [link](https://example.com)
[^html]: Using <em>HTML</em>
[^ent]: Entity support & or &amp;? < or &lt;
[^q]: "
[^long]: Sensors indicate no shuttle or other ships in this sector. According to coordinates, we have travelled 7,000 light years and are located near the system J-25. Tractor beam released, sir. Force field maintaining our hull integrity. Damage report? Sections 27, 28 and 29 on decks four, five and six destroyed. Without our shields, at this range it is probable a photon detonation could destroy the Enterprise.
[^zh]: 威廉 莎士比亚
[^🥰]: 🥰

Fixes #27387

Changes proposed in this Pull Request:

• Footnotes have a title attribute which displays a tooltip. This is now set to the contents of the footnote.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

Write a Markdown post with the following text:

Lorem ipsum dolor sit amet[^latin]. These footnotes support Markdown[^md] and HTML[^html]. They don't choke on entities[^ent]! And quote marks are fine[^q]. What about very long footnotes[^long]? Fully multilingual[^zh] and Unicode[^🥰] compatible.

[^latin]: Other Latin texts are available
[^md]: Some Markdown *emphasis* and a [link](https://example.com)
[^html]: Using <em>HTML</em>
[^ent]: Entity support & or &amp;? < or &lt;
[^q]: "
[^long]: Sensors indicate no shuttle or other ships in this sector. According to coordinates, we have travelled 7,000 light years and are located near the system J-25. Tractor beam released, sir. Force field maintaining our hull integrity. Damage report? Sections 27, 28 and 29 on decks four, five and six destroyed. Without our shields, at this range it is probable a photon detonation could destroy the Enterprise.
[^zh]: 威廉 莎士比亚
[^🥰]: 🥰

Publish the post and place your cursor over the footnote links.

[github-actions]

Thank you for your PR!

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The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

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Jetpack plugin:

• Next scheduled release: January 3, 2023.
• Scheduled code freeze: December 26, 2022.

[jeherve]

Related discussion: michelf/php-markdown#387

[anomiex]

I like the idea, but let's see what comes out of the upstream discussion. I see several good points raised there.