Add support for 'until' loops

corpus/statements.txt

@@ -53,6 +53,26 @@ done
 
 ---
 
+(program
+  (while_statement
+    condition: (command
+      name: (command_name (word))
+      argument: (word))
+    body: (do_group
+      (command name: (command_name (word)) argument: (word))
+      (command name: (command_name (word)) argument: (word)))))
+
+====================================
+Until statements
+====================================
+
+until something happens; do
+  echo a
+  echo b
+done
+
+---
+
 (program
   (while_statement
     condition: (command

grammar.js

@@ -141,7 +141,7 @@ module.exports = grammar({
     ),
 
     while_statement: $ => seq(
-      'while',
+      choice('while', 'until'),
       field('condition', $._terminated_statement),
       field('body', $.do_group)
     ),