Add async syntax

corpus/async.txt

@@ -2,7 +2,7 @@
 Simple Async Statements
 ==================
 
-async (output 'Whaddup')
+async { (output 'Whaddup') }
 
 ---
 
@@ -20,16 +20,18 @@ async (output 'Whaddup')
                     (string)))))))))))
 
 ==================
-Complex Run Statements
+Complex Async Statements
 ==================
 
-async if 1 % 2 == 0 {
+async {
+  if 1 % 2 == 0 {
   (output 'true')
   } else {
     (output 'false')
   }
 
-async (output 'foobar')
+  (output 'foobar')
+}
 
 ---
 
@@ -70,11 +72,7 @@ async (output 'foobar')
                       (tool)
                       (expression
                         (value
-                          (string)))))))))))))
+                          (string)))))))))
-  (item
-    (statement
-      (async
-        (item
           (statement
             (expression
               (function_call

grammar.js

@@ -236,6 +236,6 @@ module.exports = grammar({
       ),
     )),
 
-    async: $ => seq('async', $.item),
+    async: $ => seq('async', '{', $.item, '}'),
   }
 });

src/grammar.json

@@ -954,9 +954,17 @@
           "type": "STRING",
           "value": "async"
         },
+        {
+          "type": "STRING",
+          "value": "{"
+        },
         {
           "type": "SYMBOL",
           "name": "item"
+        },
+        {
+          "type": "STRING",
+          "value": "}"
         }
       ]
     }