Add support for `.asy` files

[tarsobcaldas]

The parser identifies the asy environment in LaTeX, and has a proper highlighting for asymptote's syntax, but the .asy files themselves don't have a parser. Is it reasonable for this feature to be included in this repository? Should be another one for itself?

[clason]

That needs to be a separate parser indeed; but you can also just use a C parser like nvim-treesitter does.