feat(formula): add formula token types

lib/src/tokens.dart

@@ -0,0 +1,44 @@
+import 'values.dart';
+
+enum TokenType {
+  number,
+  text,
+  identifier,
+  quotedName,
+  cellRef,
+  errorLit,
+  bang,
+  plus,
+  minus,
+  star,
+  slash,
+  caret,
+  percent,
+  amp,
+  eq,
+  neq,
+  lt,
+  lte,
+  gt,
+  gte,
+  lparen,
+  rparen,
+  comma,
+  colon,
+  eof,
+}
+
+class Token {
+  final TokenType type;
+  final String lexeme;
+  final Object? literal;
+
+  const Token(this.type, this.lexeme, [this.literal]);
+
+  double get numberValue => literal as double;
+  String get textValue => literal as String;
+  ErrorKind get errorKind => literal as ErrorKind;
+
+  @override
+  String toString() => 'Token(${type.name}, $lexeme)';
+}