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.

helix-loader/build.rs

@@ -0,0 +1,6 @@
+fn main() {
+    println!(
+        "cargo:rustc-env=BUILD_TARGET={}",
+        std::env::var("TARGET").unwrap()
+    );
+}