[core] fix field detection for escaped % and { format styles

[dylanpulver]

Closes #74.

parse() reads escaped literals as fields: %%(b)s in a % format, {{b}} in a { one. #69 fixed this for $; these two were left. { also folds a conversion or format spec into the name, so "{levelname:>8} {message}" — an ordinary stdlib format that StrFormatStyle.validate() accepts — logs {"levelname:>8": null, "message": "hello"}.

Changes

• %: skip %%, mirroring the $ fix.
• {: use string.Formatter, which is what logging.StrFormatStyle.validate parses the same string with. Covers {{/}}, !r, :>8, and nested specs like {levelname:>{width}}. STYLE_STRING_FORMAT_REGEX is kept but unused — say the word and I'll drop it.

Testing
pytest tests: 224 passed, 218 before. Two new tests beside the existing per-style ones. black/pylint/mypy/validate-pyproject clean.

Oracle: the substitution engine itself (fmt % tracking_dict, string.Formatter().parse, Template.get_identifiers). Over 37 formats $ agreed 7/7 as a control; %+{ diverged 13x before, 1 after — the remainder is "{}", which validate() skips too.

Reverting only the core.py change fails the 6 new cases and nothing else. A regex-only alternative passes {levelname:>8} but reads width out of {levelname:>{width}}; hence that case in the test.

One behaviour change: with validate=False a malformed { format now raises at construction instead of silently yielding no fields.

Prepared with AI assistance; I reviewed and tested it locally.