a dedicated area to write commit message (like committia)

**Is your feature request related to a problem? Please describe.**
While we can open up a vim editor to write the commit message with shortcut `C`, it blocks all the lazygit interface. I would like to be able to see the diff and maybe other stuff when I'm writing the git message.

**Describe the solution you'd like**
A dedicated area in lazygit for writing git message that contains more than just the title. It will allow me to see the diff and other stuff in the lazygit interface while I'm writing.

**Describe alternatives you've considered**
https://github.com/rhysd/committia.vim committia has what I need.