fix quadratic reindent of wide identifier lists

[alhudz]

1. ReindentFilter._process_identifierlist puts each column on its own line via insert_before(token), which resolves the position through TokenList.token_index.
2. token_index ran self.tokens[start:].index(token), copying the list tail and rescanning from 0 on every call, so an N-column list reindented in O(n^2).

repro: sqlparse.format('select ' + ','.join('a' for _ in range(16000)), reindent=True)
before: reindent step ~3.5s; a 4000-column SELECT (~8 KB, under the 10000-token cap) ~0.5s
after: reindent step ~0.12s; same 4000-column SELECT ~0.2s

fix: token_index now uses self.tokens.index(token, start) (no copy, honours the hint) and the loop keeps a forward cursor. Parse output is unchanged.