Revert "Remove now-unnecessary `avoid` when typing closures"
Revert "Remove now-unnecessary `avoid` when typing closures"

More precisely, put it under a Config flag which is off by default.

This reverts commit 629006b3e5c456d5073e5eda2fa9c39ddc32a135.

lampepfl/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty