Ensure there is a nested table allocated for #set!

Fixes the following bug:
```
test/functional/helpers.lua:107: Error executing lua:
vim/treesitter/query.lua:256: attempt to index a nil value
```

runtime/lua/vim/treesitter/query.lua

@@ -253,7 +253,11 @@ local directive_handlers = {
   ["set!"] = function(_, _, _, pred, metadata)
     if #pred == 4 then
       -- (#set! @capture "key" "value")
-      metadata[pred[2]][pred[3]] = pred[4]
+      local capture = pred[2]
+      if not metadata[capture] then
+        metadata[capture] = {}
+      end
+      metadata[capture][pred[3]] = pred[4]
     else
       -- (#set! "key" "value")
       metadata[pred[2]] = pred[3]