Such that equivalents of `trio.Cancelled` from other runtimes such as
`asyncio.CancelledError` and `subprocess.CalledProcessError` (with
a `.returncode == -2`) can be gracefully ignored as needed by the
caller.
For example this is handy if you want to avoid debug-mode REPL entry on
an exception-group full of only some subset of exception types since you
expect certain tasks to raise such errors after having been cancelled by
a request from some parent supervision sys (some "higher up"
`trio.CancelScope`, a remote triggered `ContextCancelled` or just from
and OS SIGINT).
Impl deats,
- offer a new `ignore_nested: set[BaseException]` param which by
default we add `trio.Cancelled` to when no other types are provided.
- use `ExceptionGroup.subgroup(tuple(ignore_nested)` to filter to egs of
the "ignored sub-errors set" and return any such match (instead of
`True`).
- detail a comment on exclusion case.
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