Remove Markdown-it from Content Editor build

What does this MR do?

The Content Editor component uses the prosemirror-markdown library to serialize ProseMirror documents as Markdown strings. In turn, prosemirror-markdown uses markdown-it to deserialize a Markdown strings into ProseMirror documents. We don’t deserialize Markdown strings in the Content Editor so we don’t need markdown-it.

This MR removes markdown-it from the Content Editor bundle by changing the import statement of prosemirror-markdown to only target the to_markdown module. prosemirror-markdown does not support tree-shaking.

Screenshots (strongly suggested)

This MR does not introduce user-facing changes.

With markdown-it	Without markdown-it	Size Reduction
		-33kb (from 45kb to 12kb)

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