Add the go ctags parser

Make go one of the builtin filetypes, add the parser and update the related
source and config files. While there, remove Rust from [Groups] in
filetype_extensions.conf because it's already a builtin filetype as well.

The parser itself is stolen from the fishman/ctags repo.

The parser is more on the simple side - types like structs or interfaces are not distinguished (I may have a look at it in the future if I have time, should be easy to add) but it's definitely useful for going to tag definition/declaration.

I hope I have changed all that should have been changed and haven't forgotten about anything. Everything seems to work just fine, including syntax highlighting, build commands sidebar symbols, goto tag definition/declaration.
Hum, reading the code a bit further suggest it's probably not so problematic as it's only used when reading `\n` to transform it in `TOKEN_SEMICOLON`.  So the only situation I could see it would use the previous run's value is if the first line was empty, but then all it would do is report an extra semicolon token before anything else -- I doubt this would break anything.