migrate grammar fetching/building code into 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.

Cargo.toml

@@ -6,6 +6,7 @@ members = [
   "helix-tui",
   "helix-lsp",
   "helix-dap",
+  "helix-loader",
   "xtask",
 ]