This commit also includes a basic proof-of-concept demo for creating vim
statuslines. When creating vim statuslines you basically need to first
create the statusline the same way as the terminal demo, use the vim
renderer and then afterwards loop through the collected highlight groups
in the vim renderer and write those as vim highlight statements.
Vim obviously needs a ton of wrapper code to make everything work
properly (separate modes, callbacks, all that stuff) so the current demo
only shows some basic statusline highlighting.
