Markdown editor with Rich Text preview via MDAST

[Soxasora]

Description

Based on #2501
This is the first stage of the transition to a new Lexical-based editor.

Introduces a Lexical plain text editor with a rich text preview, using MDAST to map and transform Lexical nodes to markdown, aiming to provide lossless transformations.

This PR focuses on providing the same editor experience as before, while transitioning to Lexical.

Screenshots

Additional Context

Persisting and handling the tree in the database is still something that's being conceptualized, but like the original PR, we'll persist the lexical JSON state and its HTML variant.

WIP: More infos on MDAST can be found in docs/dev/lexical-mdast.md

Checklist

Are your changes backward compatible? Please answer below:
Yes, with a distributed migration approach we're going to migrate legacy content while still keeping its original version right in the text field.

On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:
5 - in dev with iterative QA

For frontend changes: Tested on mobile, light and dark mode? Please answer below:
In QA

Did you introduce any new environment variables? If so, call them out explicitly here:
n/a

Did you use AI for this? If so, how much did it assist you?
Lexical: Ask mode to bridge the gaps in the documentation
MDAST: Light usage of Agent mode for refactoring and ensuring every transformer is well-represented

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	@​lexical/​clipboard@​0.38.2
	@​lexical/​rich-text@​0.38.2
	@​lexical/​selection@​0.38.2
	@​lexical/​utils@​0.38.2
	@​lexical/​code-shiki@​0.38.2
	@​lexical/​history@​0.38.2
	@​lexical/​html@​0.38.2
	dompurify@​3.3.0
	linkedom@​0.18.12
	@​lexical/​headless@​0.38.2
	katex@​0.16.11 ⏵ 0.16.25		+8	+1
	@​lexical/​react@​0.38.2
	lexical@​0.38.2

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