[mlir][PDLL] Tweak the grammar to highlight partial code better

This commit enables proper highlighting when inner statements are
outside of a constraint/pattern/etc. This shouldn't really happen in
actual code, but can happen in documentation (which uses the same
syntax grammar).

mlir/utils/vscode/pdll-grammar.json

@@ -27,6 +27,9 @@
     },
     {
       "include": "#pattern"
+    },
+    {
+      "include": "#inside_pattern"
     }
   ],
   "repository": {