Make the mccabe checker ignore the case of a single top-level elif chain or match statement

This is a somewhat common edge case where there shouldn't be a need to use noqa comments. Non-looping functions with a nesting depth of 1 should be excluded from the mccabe checker, or there should be a separate error code for this particular kind of pattern if you want to lint or ignore it separately, such as "elif chain exceeds mccabe complexity".

Incidentally, while discussing code complexity KPIs, being able to set a maximum allowed nesting depth would be nice.

Start by locating Ruff's mccabe checker and its handling of top-level elif chains and match statements. Determine how non-looping functions with nesting depth 1 are currently reported, then verify the chosen behavior for ignoring them or assigning a separate error code; the requested maximum nesting-depth setting is additional scope.

Written by the indexing model from the issue text.