Handle

Fixes #39

corpus/literals.txt

@@ -41,7 +41,7 @@ echo $abc
 Special variable expansions
 =============================
 
-echo $# $* $@
+echo $# $* $@ $!
 
 ---
 
@@ -50,6 +50,7 @@ echo $# $* $@
     (command_name (word))
     (simple_expansion (special_variable_name))
     (simple_expansion (special_variable_name))
+    (simple_expansion (special_variable_name))
     (simple_expansion (special_variable_name))))
 
 =============================

grammar.js

@@ -450,6 +450,7 @@ module.exports = grammar({
       choice(
         $._simple_variable_name,
         $._special_variable_name,
+        alias('!', $.special_variable_name),
         alias('#', $.special_variable_name)
       )
     ),

src/grammar.json

@@ -1884,6 +1884,15 @@
               "type": "SYMBOL",
               "name": "_special_variable_name"
             },
+            {
+              "type": "ALIAS",
+              "content": {
+                "type": "STRING",
+                "value": "!"
+              },
+              "named": true,
+              "value": "special_variable_name"
+            },
             {
               "type": "ALIAS",
               "content": {