This PR globally enables `warning.simp.varHead` (added in #13325) and
silences the warning in `Lake.Util.Family.Mathlib` adaptations were
already merged as part of adaptations for #13325. This is a separate PR
from #13325 due to warning appearing when re-bootstrapping, so we needed
`stage0` update before enabling this option.
This PR adds warnings when registering `@[simp]` theorems whose
left-hand side has a problematic head symbol in the discrimination tree:
- **Variable head** (`.star` key): The theorem will be tried on every
`simp` step, which can be expensive. The warning notes this may be
acceptable for `local` or `scoped` simp lemmas. Controlled by
`warning.simp.varHead` (default: `true`).
- **Unrecognized head** (`.other` key, e.g. a lambda expression): The
theorem is unlikely to ever be applied by `simp`. Controlled by
`warning.simp.otherHead` (default: `true`).
