use autocmd! so that *.fs is not forth AND glsl

*.fs is setf forth in filetype.vim
https://github.com/nschloe/vim/blob/3cd009196341948f0730fe9f42859660bf759093/runtime/filetype.vim#L751-L752

Forth syntax sets iskeyword to very permissive values, since I think pretty much all non-whitespace is valid in forth identifiers.

This commit overwrites *.fs to be glsl only.