Add markedit-paste-markdown

[lbwork5388]

Adds markedit-paste-markdown, which converts rich text (HTML) on the clipboard to Markdown on paste.

How it differs from Enhanced Paste:

• Only converts HTML with real structure. Code copied from editors (VS Code, Cursor) arrives as styled <div>/<span> and is pasted as plain text, with indentation kept.
• Doesn't convert when the cursor is inside a code block or inline code.
• GFM tables, strikethrough, and task lists (via turndown-plugin-gfm).
•   becomes a normal space.
• Light escaping: MY_CONST stays as-is, while text that would turn into Markdown syntax is still escaped.

Checks:

• The release asset is built by a GitHub Actions workflow from the tagged source, after typecheck and tests.
• yarn test and yarn build (with integrity check) pass locally.

[cyanzhong]

Thank you for your contribution.

How is this different from https://github.com/MarkEdit-app/MarkEdit-enhanced-paste?

I would suggest improving the existing extension if this one is not hugely different.