Support Vim's format paragraph `gq` command

**Describe the solution you'd like**

It would be awesome if HackMD supported Vim's [gq](https://vimdoc.sourceforge.net/htmldoc/usr_25.html) command.

Start with Vim's gq documentation linked in the issue to understand the requested command and its formatting behavior. Then locate HackMD's editor integration and determine how keyboard commands are handled. Done means the editor supports gq consistently with the documented Vim behavior, with coverage for the relevant formatting cases.

索引モデルが issue の本文から書いたものです。