* replace `immutable` with `constant` for _PERMIT_TYPEHASH

This commit is related to the following issue discussion: https://github.com/OpenZeppelin/contracts-wizard/issues/89#issuecomment-1042391318

Since Solidity version `0.6.12` the `keccak256` of string literals is treated specially and the hash is evaluated at compile time. Since the OpenZeppelin Wizard also uses `constant` for OpenZeppelin's AccessControl's roles declarations, it's good practice to make this consistent.

* Update CHANGELOG

* fix: ensure transpiler compatibility

* fix: fixing var-name-mixedcase

* prettier & lint check

Signed-off-by: Pascal Marco Caversaccio <pascal.caversaccio@hotmail.ch>

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>