PHP 8.4 compatibility and tooling improvements

[jackbayliss]

Changes

• Adds ext-mbstring to requirements, as we use various mb_ methods & update composer lock
• Adds --parallel to pint cs-fix
• Because it's PHP 8.4, we no longer need to wrap new calls in parentheses
• Adds a few string types to consts
• Removes an is_string check as strict comparison is used and switches it for ||

Noticed these while poking about!

Setting as draft as may come back, may not.

[ziadoz]

Seems fine to merge this to me.

[weshooper]

Thanks!