feat: multiple grind propagators per declaration

[leodemoura]

This PR allows users to declare additional grind constraint propagators for declarations that already include propagators in core.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 7ee3079afbdb893b124068078f654269be09a95b --onto ac0b82933f6eac9914011ca2caf38d0e4e991160. You can force Mathlib CI using the force-mathlib-ci label. (2025-09-27 00:28:31)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 7ee3079afbdb893b124068078f654269be09a95b --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-09-27 00:28:33)