fetch grammars by URL

[the-mikedavis]

I've been working on improving packaging in https://github.com/helix-editor/tspm and IMO it's pretty close to being usable for all grammars.

For most grammars, pulling from TSPM is not a big difference. It's just a smaller/faster download.

For some oddly shaped grammars like tree-sitter-ocaml or tree-sitter-typescript (multiple grammars in one repository), it helps reduce the complexity of the build: we wouldn't need the subpath config.

Other grammars like tree-sitter-rescript or alex-pinkus/tree-sitter-swift (don't store generated C files in the git repo, #1964) would benefit more concretely. We wouldn't need to maintain any forks that check the generated C into the repo.

As a bonus for Nix packaging, the pkgs.tspm.io links contain the SHA256 hash of the file, which we can use it to construct a fixed-output-derivation (FODs can be cached infinitely which solves #1779).

The [[grammar]] config in languages.toml should accept a new source which would be used like so:

[[grammar]]
name = "elixir"
source = { url = "https://pkgs.tspm.io/elixir/elixir-lang/a11a686303355a518b0a45dea7c77c5eebb5ec22-0.20.6-13-956eb868f38544dedcd5ef45f0ecc7cab542c68b89fae631e149008ad5cc72e8-src.tar.gz" }

Implementation-wise we have two kinda bad options:

1. take a dependency on a Rust HTTP client
2. take a runtime dependency on curl or similar

I lean towards (2). So we'd be switching from having a runtime dependency on git to curl. That's not super ideal but I think it's acceptable: you only need curl if you want to hx -g fetch, and package-managers pre-fetch and build grammars, so most users will not need to care about this at all.