Showing mentions in markdown in the composer

[artcodespace]

Relates to: https://github.com/vector-im/verticals-internal/issues/96

Work is currently being done to change over mentions from being immutable links to being their own type, a MentionNode.

When we did this, we decided that a mention, when output as markdown, should be output as a plain text mxId. This causes issues for iOS which was previously using the link type markup [link text](link url) to determine what should be represented as a pill.

We think it should be possible to format a mention as a pill (ie a tag looking something like <a contenteditable="false" data-mention-type="user">...</a>) and insert this inside the markdown.

This probably means that we would require two markdown outputs, similar to what we are working on for html. Likely lib functions would be functions that allow us to:

• output html in the composer format
• output html in the matrix message format
• output markdown in the composer format (ie with mentions formatted as html)
• output markdown in the matrix message format