[native] Introduce TextMessageMarkdownContext

Summary: We need the AST from inside InnerTextMessage in order to determine whether the Markdown has any "pressables" (link or spoiler for now, but @mention in the future).

Test Plan: Flow, tested with following diff

Reviewers: rohan, atul, ginsu, michal

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D5754