Use explicit 'nil_literal' rule

Makes the dumped AST more readable, and will probably help in writing
future queries.

grammar.js

@@ -26,7 +26,7 @@ module.exports = grammar({
     ),
 
     _expr: ($) => choice(
-      "nil",
+      $.nil_literal,
       $.integer_literal,
       $.string_literal,
 
@@ -35,6 +35,8 @@ module.exports = grammar({
       $.sequence_expression,
     ),
 
+    nil_literal: (_) => "nil",
+
     integer_literal: (_) => /[0-9]+/,
 
     string_literal: ($) => seq(