feat(web-pkg): support markdown YAML frontmatter in the text editor

[kulmann]

Description

Adds a frontmatter custom node and integrates it into markdown toolbar and slashcommand-menu:

• Frontmatter only valid on markdown content
• Frontmatter can be toggled off (= delete with confirmation) or on (= if already present: jump cursor into the box, if not present yet: create one at the top of the document)
• pressing delete in front of or after the frontmatter just moves the cursor into the frontmatter-box to avoid accidental deletion
• all the markdown goodies are disabled when the cursor is in the frontmatter-box, only plain text (and emojis) are allowed in there
• frontmatter content is not interpreted as YAML (at least not in this PR for the sake of simplicity), just rendered and manipulated as plain text

Related Issue

• Fixes text-editor corrupts YAML frontmatter on save — even when nothing was edited #3256

How Has This Been Tested?

• unit tests
• manually standalone
• manually with y.js session

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)

[JammingBen]

One somewhat critical finding: having something between two --- inside a quote gets erased. E.g. a markdown file with:

> ---
> a: 1
> ---

Foo

Will result in Foo.

Some smaller findings, still worth looking into IMO:

• Keybinding keep working inside frontmatter. E.g. ctrl/cmd + alt + 1 destroys the frontmatter and makes it a headline.
• Claude found that not passing a custom marked instance to MarkdownManager will make it use a singleton. This singleton gets appended every extension carrying a markdownTokenizer and never gets cleaned up. This leads to more and more tokenizers getting appended and running. The solution is to create a marked instance ourselves per strategy and pass it to the MarkdownManager (and possibly the markdown extension?).

[AlexAndBear]

😍