[perf] Make PrettyPrinter format lazily so output can be budget-capped (pytest-dev#14588)

``_format`` and the per-type helpers now ``yield`` their output as a stream of string chunks instead of writing to a file-like object, and ``pformat`` joins them. On top of that, ``pformat_lines`` pulls from the formatter only until a budget is reached:

    pformat_lines(obj, max_lines=None, max_chars=None)

It stops on the first chunk that reaches *either* budget, so a huge collection costs O(budget) rather than O(N). Either dimension may be ``None`` (unbounded); with both ``None`` the whole object is formatted.

Benchmark (``PrettyPrinter`` alone, width 80)::

    list(range(500_000)):
        pformat().splitlines()        ~805 ms
        pformat_lines(max_lines=11)   ~0.027 ms      (~30000x)

    [8 small ints] (common small diff):
        pformat().splitlines()        ~0.0133 ms
        pformat_lines(max_lines=11)   ~0.0163 ms (+3µs)

    ["x"*100_000] * 3 (flat, few huge elements):
        pformat_lines(max_chars=640)  stops after ~100_000 chars
                                      (one element) instead of 300_000

Author: Pierre-Sassoulas