add support for generating parser.c in tree-sitter grammars

not all tree-sitter repos ship the generated file, so we may have to
generate it ourself.

helix-loader/src/grammar.rs

@@ -395,6 +395,20 @@ fn build_tree_sitter_library(
     let parser_path = src_path.join("parser.c");
     let mut scanner_path = src_path.join("scanner.c");
 
+    if !parser_path.exists() {
+        let mut command = Command::new("tree-sitter");
+        command.arg("generate");
+        command.current_dir(src_path.parent().unwrap());
+        let output = command.output().context("Failed to execute tree-sitter")?;
+        if !output.status.success() {
+            return Err(anyhow!(
+                "tree-sitter parser generation failed.\nStdout: {}\nStderr: {}",
+                String::from_utf8_lossy(&output.stdout),
+                String::from_utf8_lossy(&output.stderr)
+            ));
+        }
+    }
+
     let scanner_path = if scanner_path.exists() {
         Some(scanner_path)
     } else {