This is an optimization for the release CI. The release CI can take
a while since it compiles release builds for all operating systems.
We cut down on duplicate work and overall time by fetching
tree-sitter grammar repositories and then using those repositories
in all later steps. Previously we built all of helix just to run
helix_loader::grammar::fetch_grammars()
which is wasteful on time. With this change we only build the
helix-loader crate.
This is a rather large refactor that moves most of the code for
loading, fetching, and building grammars into a new helix-loader
module. This works well with the [[grammars]] syntax for
languages.toml defined earlier: we only have to depend on the types
for GrammarConfiguration in helix-loader and can leave all the
[[language]] entries for helix-core.
