sh.yaml: Match parameter expansions with braces using \w

Author: niten94

runtime/syntax/sh.yaml

@@ -41,7 +41,7 @@ rules:
       # Conditional flags
     - statement: "\\s+(-[A-Za-z]+|--[a-z]+)"
 
-    - identifier: "\\$\\{[0-9A-Za-z_:!%&=+#~@*^$?, .\\-\\/\\[\\]]+\\}"
+    - identifier: "\\$\\{[\\w:!%&=+#~@*^$?, .\\-\\/\\[\\]]+\\}"
     - identifier: "\\$([0-9_!#@*$?-]|[A-Za-z_]\\w*)"
 
     - constant.string: