Add support for comment highlighting

This will be supported by the comment extension. If the
extension is not installed, then this would be a no-op.

Signed-off-by: Donnie Adams <donnie@obot.ai>

Author: thedadams

languages/dockerfile/injections.scm

@@ -1,6 +1,5 @@
-; We need impl this
-; ((comment) @injection.content
-;  (#set! injection.language "comment"))
+((comment) @content
+ (#set! injection.language "comment"))
 
 ((shell_command) @content
  (#set! "language" "bash"))