Add builtin -q

Used to query for a builtin's existence, like `type -q` and `functions
-q` can be used to query for a things and a functions existence respectively.

tests/test_builtinbuiltin.in

@@ -0,0 +1,7 @@
+# Tests for the "builtin" builtin/keyword.
+builtin -q string; and echo String exists
+builtin -q; and echo None exists
+builtin -q string echo banana; and echo Some of these exist
+builtin -nq string
+echo $status
+exit 0