downcase! directive to use with ruby injections

```
(#downcase! "language")
```

downcase! will ensure the metadata value for the specified key will be
downcased. If the value is a node, it will downcase the text specified
by the node.

```
(#downcase! @node "key")
```

You can also namespace the key with a specific capture, similar to how
you can call `(#set! @node "key" "value")`

lua/nvim-treesitter/query_predicates.lua

@@ -87,3 +87,30 @@ end)
 
 -- Just avoid some anoying warnings for this directive
 query.add_directive('make-range!', function() end)
+
+query.add_directive('downcase!', function(match, _, bufnr, pred, metadata)
+  local text, key, value
+
+  if #pred == 3 then
+    -- (#downcase! @capture "key")
+    key = pred[3]
+    value = metadata[pred[2]][key]
+  else
+    -- (#downcase! "key")
+    key = pred[2]
+    value = metadata[key]
+  end
+
+  if type(value) == "string" then
+    text = value
+  else
+    local node = match[value]
+    text = query.get_node_text(node, bufnr)
+  end
+
+  if #pred == 3 then
+    metadata[pred[2]][key] = string.lower(text)
+  else
+    metadata[key] = string.lower(text)
+  end
+end)

queries/ruby/injections.scm

@@ -1 +1,7 @@
 (comment) @comment
+
+(heredoc_body
+ (heredoc_content) @content
+ (heredoc_end) @language
+ (#set! "language" @language)
+ (#downcase! "language"))