grammar: support highlighting ${variables} for e.g. record_transformer

Related to adamchainz/fluentd.tmLanguage#4

grammars/fluentd.cson

@@ -149,6 +149,10 @@ repository:
         match: '\\b(\\d+[smhd])\\b'
         name: 'value.constant.time.fluentd'
       }
+      {
+        match: '(\\$\\{[^\\}]*\\})'
+        name: 'variable.string.fluentd'
+      }
       {
         match: '([^\\s]+?)'
         name: 'meta.string.unquoted.fluentd'