feat: allow external scanners to use the logger

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

test/fixtures/test_grammars/external_tokens/scanner.c

@@ -77,6 +77,7 @@ bool tree_sitter_external_tokens_external_scanner_scan(
 
     for (;;) {
       if (scanner->depth == 0) {
+        lexer->log(lexer, "Found a percent string");
         lexer->result_symbol = percent_string;
         return true;
       }