scanner: add identifiers

src/new_scanner.zig

@@ -150,12 +150,21 @@ pub const Scanner = struct {
         return self.makeToken(.NUMBER);
     }
 
+    fn doIdentifier(self: *Scanner) Token {
+        while (isAlphaNumeric(self.peek())) {
+            _ = self.advance();
+        }
+
+        return self.makeToken(.IDENTIFIER);
+    }
+
     pub fn scanToken(self: *Scanner) !?Token {
         self.skipWhitespace();
         self.start = self.current;
         if (self.isAtEnd()) return self.makeToken(TokenType.EOF);
 
         var c = self.advance();
+        if (isAlpha(c)) return self.doIdentifier();
         if (isDigit(c)) return self.doNumber();
 
         var token = switch (c) {