Emit warning when`cjsInterop` is enabled without `splitting`

Either emit a warning when`cjsInterop` is enabled without `splitting`. Another option might be to automatically enable `splitting` when `cjsInterop` is enabled. But otherwise `cjsInterop` has no effect and it is unclear why.

The issue does not name a file or test. Start by tracing where the cjsInterop and splitting options are handled, then check how existing option warnings are tested. Done means users are warned when cjsInterop is enabled without splitting, or the project consistently enables splitting automatically, with the chosen behavior documented by tests.

Written by the indexing model from the issue text.