Add GAP grammars to nvim-treesitter

[reiniscirpons]

nvim-treesitter is a neovim plugin which bundles language grammars for simple installation. Adding our grammars to nvim-treesitter would simplify installation in neovim, see also #2 .

To add our grammars we need to:

• Fork nvim-treesitter
• Add languages to lua/nvim-treesitter/parsers.lua, and queries to queries/gap, queries/gaptst in the nvim-treesitter directory.
• Make sure query formatting and used captures match the ones given in the contributing guide.
• Add fold query file
• Add indent query file (this is a bit finicky at the moment so we are skipping it).
• Make a PR to nvim-treesitter (feat(gap): Add gap and gaptst parser and queries nvim-treesitter/nvim-treesitter#7110).
• Address any comments.

I am currently working on this, this issue serves as a progress tracker.