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

Related to adamchainz/fluentd.tmLanguage#4

CHANGELOG.md

@@ -9,6 +9,7 @@ NEW FEATURES:
 - Support escape sequenes like `foo\nbar` in single, double and unquoted strings
 - Support array and hash (using JSON source) value types like `["foo","bar"]`
 - Support integer, time and size value types (e.g. `property 100`, `bufsize 5M` or `finterval 5s`)
+- Support highlighting ${variables} (used by e.g. [record_transformer plugin](https://docs.fluentd.org/v0.12/articles/filter_record_transformer))
 
 IMPROVEMENTS:
 

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'