Disable keyword extraction

For now bare keywords are errors, since we have not included them as
valid expressions.

grammar.js

@@ -17,6 +17,9 @@ const PREC = {
 module.exports = grammar({
   name: "tiger",
 
+  // Ensure we don't extract keywords from tokens
+  word: ($) => $.identifier,
+
   rules: {
     source_file: ($) => choice(
       $._expr,
@@ -40,6 +43,9 @@ module.exports = grammar({
       '"',
     ),
 
+    // NOTE: includes reserved identifiers
+    identifier: (_) => /[_a-zA-Z0-9]+/,
+
     escape_sequence: (_) => token.immediate(
       seq(
         "\\",