Problem: The default commentstring for C/C++ can lead to invalid code
when commenting and does not match the Nvim codebase.
Solution: Change commentstring to `// %s` as used by Neovim. Also
set all commentstrings that derive from the default C string explicitly
(and correctly).
Problem: No commentstring is set for C# buffers after removing the
default C-style commentstring
Solution: Add `ftplugin/cs.lua` with C-style commentstring
