Add support for strings ending with $ (#24)

corpus/literals.txt

@@ -252,10 +252,12 @@ Strings containing special characters
 
 echo "s/$/'/"
 echo "#"
+echo "s$"
 
 ---
 
 (program
+  (command (command_name (word)) (string))
   (command (command_name (word)) (string))
   (command (command_name (word)) (string)))
 

grammar.js

@@ -389,6 +389,7 @@ module.exports = grammar({
         ),
         optional($._concat)
       )),
+      optional('$'),
       '"'
     ),
 

src/grammar.json

@@ -1570,6 +1570,18 @@
             ]
           }
         },
+        {
+          "type": "CHOICE",
+          "members": [
+            {
+              "type": "STRING",
+              "value": "$"
+            },
+            {
+              "type": "BLANK"
+            }
+          ]
+        },
         {
           "type": "STRING",
           "value": "\""