parsers: add tree-sitter-gitignore

[theHamsta]

Not feeling too confident about the captures yet. Any feedback welcome!

@andy2mrqz

Addresses #2282 (comment)

[theHamsta]

Please note that gitignore is not a known filetype to vim. You need to add something like

augroup filetypedetect
    au! BufRead,BufNewFile .gitignore set filetype=gitignore
augroup end

to your init.vim. Support for the filetype should be contributed to github.com/vim/vim

[shunsambongi]

@theHamsta I was planning on contributing this parser eventually, but I had become a little busy, so I appreciate this PR. I don't know if it would help you at all, but I was playing around with some highlight queries in https://github.com/shunsambongi/gitignore.nvim/blob/0b8108524dbc0d15a34b6e1ca5bbd75b04a5ddfb/queries/gitignore/highlights.scm

[shunsambongi]

Please note that gitignore is not a known filetype to vim. You need to add something like

augroup filetypedetect
    au! BufRead,BufNewFile .gitignore set filetype=gitignore
augroup end

to your init.vim. Support for the filetype should be contributed to github.com/vim/vim

I also just wanted to point out that there are a bunch of tools that use ignore files that can be highlighted using the parser being added in this PR. For example, sharkdp/fd (.fdignore), BurntSushi/ripgrep (.rgignore), prettier (.prettierignore), etc. So one would just need to modify the autocmd to support these files.

[theHamsta]

Commited your changes