feat: add 'reserved word' construct

Co-authored-by: Amaan Qureshi <amaanq12@gmail.com>

test/fixtures/test_grammars/reserved_words/corpus.txt

@@ -0,0 +1,101 @@
+==============
+Valid Code
+==============
+
+if (a) {
+  var b = {
+    c: d,
+    e: f,
+  };
+  while (g) {
+    h();
+  }
+}
+
+---
+
+(program
+  (if_statement
+    (parenthesized_expression (identifier))
+      (block
+        (var_declaration
+          (identifier)
+          (object
+            (pair (identifier) (identifier))
+            (pair (identifier) (identifier))))
+        (while_statement
+          (parenthesized_expression (identifier))
+          (block (expression_statement (call_expression (identifier))))))))
+
+================================================
+Error detected at globally-reserved word
+================================================
+
+var a =
+
+if (something) {
+  c();
+}
+
+---
+
+(program
+  (ERROR (identifier))
+  (if_statement
+    (parenthesized_expression (identifier))
+    (block
+      (expression_statement (call_expression (identifier))))))
+
+================================================
+Object keys that are reserved in other contexts
+================================================
+
+var x = {
+  if: a,
+  while: b,
+};
+
+---
+
+(program
+  (var_declaration
+    (identifier)
+    (object
+      (pair (identifier) (identifier))
+      (pair (identifier) (identifier)))))
+
+================================================
+Error detected at context-specific reserved word
+================================================
+
+var x = {
+var y = z;
+
+---
+
+(program
+  (ERROR (identifier))
+
+  ; Important - var declaration is still recognized,
+  ; because in this example grammar, `var` is a keyword
+  ; even within object literals.
+  (var_declaration
+    (identifier)
+    (identifier)))
+
+=============================================
+Other tokens that overlap with keyword tokens
+=============================================
+
+var a = /reserved-words-should-not-affect-this/;
+var d = /if/;
+
+---
+
+(program
+  (var_declaration
+    (identifier)
+    (regex (regex_pattern)))
+  (var_declaration
+    (identifier)
+    (regex (regex_pattern))))

test/fixtures/test_grammars/reserved_words/grammar.js

@@ -0,0 +1,67 @@
+const RESERVED_NAMES = ["if", "while", "var"];
+const RESERVED_PROPERTY_NAMES = ["var"];
+
+module.exports = grammar({
+  name: "reserved_words",
+
+  reserved: {
+    global: $ => RESERVED_NAMES,
+    property: $ => RESERVED_PROPERTY_NAMES,
+  },
+
+  word: $ => $.identifier,
+
+  rules: {
+    program: $ => repeat($._statement),
+
+    block: $ => seq("{", repeat($._statement), "}"),
+
+    _statement: $ => choice(
+      $.var_declaration,
+      $.if_statement,
+      $.while_statement,
+      $.expression_statement,
+    ),
+
+    var_declaration: $ => seq("var", $.identifier, "=", $._expression, ";"),
+
+    if_statement: $ => seq("if", $.parenthesized_expression, $.block),
+
+    while_statement: $ => seq("while", $.parenthesized_expression, $.block),
+
+    expression_statement: $ => seq($._expression, ";"),
+
+    _expression: $ => choice(
+      $.identifier,
+      $.parenthesized_expression,
+      $.call_expression,
+      $.member_expression,
+      $.object,
+      $.regex,
+    ),
+
+    parenthesized_expression: $ => seq("(", $._expression, ")"),
+
+    member_expression: $ => seq($._expression, ".", $.identifier),
+
+    call_expression: $ => seq($._expression, "(", repeat(seq($._expression, ",")), ")"),
+
+    object: $ => seq("{", repeat(seq(choice($.pair, $.getter), ",")), "}"),
+
+    regex: $ => seq('/', $.regex_pattern, '/'),
+
+    regex_pattern: $ => token(prec(-1, /[^/\n]+/)),
+
+    pair: $ => seq(reserved('property', $.identifier), ":", $._expression),
+
+    getter: $ => seq(
+      "get",
+      reserved('property', $.identifier),
+      "(",
+      ")",
+      $.block,
+    ),
+
+    identifier: $ => /[a-z_]\w*/,
+  },
+});