Use a minimal binary to fetch grammar sources in release CI (#2557)

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.

helix-loader/src/main.rs

@@ -0,0 +1,9 @@
+use anyhow::Result;
+use helix_loader::grammar::fetch_grammars;
+
+// This binary is used in the Release CI as an optimization to cut down on
+// compilation time. This is not meant to be run manually.
+
+fn main() -> Result<()> {
+    fetch_grammars()
+}