Add support for the roff language

[lgehr]

Hello, I write documents with the typesetter groff, which uses the roff language introduced in the original unix. I can not use ltex-ls for this.

Describe the solution you'd like I would like for ltex-ls to support groff files. This should be easy to implement, given the grammar of the language is quite simple.

Describe alternatives you've considered None

Additional context The grammar of the language is quite simple. In a nutshell:

• Lines starting with a period "." are control lines and can be ignored by ltex-ls
• All other lines should be checked. If a backslash "" in the beginning of a word is found the word is ignored.
• " starts a comment -> The rest of the line can be ignored.
• Lines between so called preprocessor macros can be ignored. More details can be found in groff(7)

The filexetensions should probably be: .roff, .troff, .groff, .nroff

Thanks!

[Kuchteq]

This would be wonderful addition! Also .ms file extension.