Julia queries: limit markdown highlighting to docstrings

runtime/queries/julia/injections.scm

@@ -1,5 +1,18 @@
+(
+  (source_file
     (
       (string_literal) @injection.content
+      [
+        (module_definition)
+        (function_definition)
+        (macro_definition)
+        (primitive_definition)
+        (abstract_definition)
+        (struct_definition)
+        (assignment_expression)
+        (const_statement)
+      ]
+    ))
   (#set! injection.language "markdown"))
 
 (