Merge yabridge-group.exe and yabridge-host.exe

They were 95% the same, so this makes a lot more sense this way. When
group host processes were introduced yabridge only did VST2 bridging,
but we already have a plugin type argument anyways so might as well
reuse that for group hosts.

ROADMAP.md

@@ -23,8 +23,6 @@ page lists some of those.
 - Replace the use of `notify-send` for notifications with using `libdbus`
   directly. Most systems will have both available by default, but some less
   common distros split `notify-send` from the rest of the `libnotify` package.
-- Possibly combine the `yabridge-host` and `yabridge-group` binaries to save
-  some disk space as 95% of their code overlaps.
 - Consider adding an option for yabridgectl to set up VST2 plugins in `~/.vst`.
   As discussed in a couple places already doing so would come with a number of
   downsides and potential pitfalls so this may not happen.