Replace Trix with Lexxy

Lexxy is an ActionText frontend replacement for Trix that has a few advantages:
1. It supports WYSIWYG markdown editing (closes https://github.com/basecamp/once-campfire/pull/36)
2. It supports inline and block code blocks with syntax highlighting

While the Ruby side of replacing Trix with Lexxy is straight forward, a lot of the Stimulus controllers will have to be updated to work with Lexxy - like the composer, the unfurler, ...

Start by locating the ActionText integration and the Stimulus controllers that handle the composer and unfurler. Review how those controllers currently interact with Trix, then trace the Ruby-side replacement and Lexxy’s markdown and code-block behavior. Done means Trix is replaced by Lexxy and the affected controllers support the new editor, including unfurling and code highlighting.

Written by the indexing model from the issue text.