[Feat] Add `keepAlive` option to `<UModal>`

[ShayanTheNerd]

Package

v4.x

Description

It’d be nice to have something like <UModal :keepAlive="true"></UModal> to retain the state of the modal (prevent it from unmounting its content).
This could come in handy when we have, for example, a login/sign-up form within a modal, so the user doesn't lose input values, etc., when closing and re-opening the modal.

[benjamincanac]

The Modal component would need an :unmount-on-hide="false" prop like on other components but it's missing in Reka UI 😬