add Support for converting footnotes markdown into block

[USERSATOSHI]

What?

Closes #57152
Depends on: #70615

This PR adds support to convert footnotes in markdown to footnotes block.

Why?

Right now, When pasting markdown containing footnotes, the footnotes are not converted into footnotes block by showdown.

How?

This PR adds that support by extending showdown to parse footnotes in markdown to html and lets gutenberg's htmlToBlock function handle the conversion with added support for updating the meta of that post.

This adds supports for raw transform in footnotes, updating htmlToBlocks to take account for footnotes content and extend showdown package to add footnotes parser.

Testing Instructions

1. Open a post or page.
2. Copy the following markdown

This is a statement that needs a citation.[^1] And here’s another interesting fact.[^2]

More content here. You can keep writing as usual.

[^1]: This is the first footnote. You can include a link or detailed explanation here.
[^2]: This is the second footnote. Additional info goes here.

3. Paste the content in the post
4. See footnotes block being generated from markdown.

Screenshots or screencast

Screen.Recording.2025-07-08.at.12.03.54.AM.mov

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Co-authored-by: USERSATOSHI <[email protected]>
Co-authored-by: zdenys <[email protected]>
Co-authored-by: audunmb <[email protected]>
Co-authored-by: stronenv <[email protected]>

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