This PR is just one part of item MAS-86 on our public roadmap [1] "Markdown formatting for posts" which is meant to enable users to use code spans/blocks and bold and italic text as championed by the Elk third-party app [2]. I believe "rich text" in the PR title is a little not nuanced enough because like before I'm of the position that Mastodon is the wrong medium for long-form articles and the various block-level formatting structures that come with it, but allowing users to use bold text without resorting to mathematical Unicode characters is a win for accessibility.
[1]: https://joinmastodon.org/roadmap [2]: https://elk.zone