Add support for omnisharp language server
It would be nice to have support for the omni-sharp language server now that we have a mono5/c♯/gtk♯ template.
We should be able to re-use a great deal of the language server infrastructure already in Builder.
It might be beneficial to ship the language server in the flatpak mono5 sdk extension, so that there is nothing to install/manage on our side of things. It would also have the benefit of ensuring the language server runs inside of the development runtime.