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(
         "\\",

src/grammar.json

@@ -1,5 +1,6 @@
 {
   "name": "tiger",
+  "word": "identifier",
   "rules": {
     "source_file": {
       "type": "CHOICE",
@@ -72,6 +73,10 @@
         }
       ]
     },
+    "identifier": {
+      "type": "PATTERN",
+      "value": "[_a-zA-Z0-9]+"
+    },
     "escape_sequence": {
       "type": "IMMEDIATE_TOKEN",
       "content": {

test/corpus/identifiers.txt

@@ -0,0 +1,21 @@
+================================================================================
+_main identifier
+================================================================================
+
+_main
+
+--------------------------------------------------------------------------------
+
+(ERROR
+  (identifier))
+
+================================================================================
+Normal identifier
+================================================================================
+
+my_1st_variable
+
+--------------------------------------------------------------------------------
+
+(ERROR
+  (identifier))