Add new syntax to let the mypy test file syntax anticipate --pretty output

The thing that makes #19510 complex is the differences in test output. One possible approach to this is to update the test output to be what it will be if --pretty is enabled. This is complicated by the fact that our current syntax for inline errors in the test data does not actually support what --pretty outputs. Adding that feature could possibly be useful.

(Additionally, https://github.com/python/mypy/issues/19940 should probably be fixed, and https://github.com/python/mypy/pull/19546 merged, before this issue is addressed, if one wants the process to be maximally smooth.)

Huh, it hadn't even occurred to me — there's no syntax for multi-line inline comments, is there? Nor for ignoring subsequent lines of a pretty comment in a test...

Originally posted by @wyattscarpenter in https://github.com/python/mypy/issues/19510#issuecomment-3140992574

Inizia leggendo la sintassi esistente dei test inline-error e le discussioni in #19510, #19940 e PR #19546 per comprendere i vincoli previsti. Definisci e implementa una sintassi per un output pretty su più righe, incluso l'ignorare le righe successive, quindi verifica che i casi di test mypy pertinenti possano esprimere e convalidare l'output di --pretty.