[collapsible][accordion] Add keepMounted prop

[mj12albert]

Closes #728

We decided to default keepMounted to false for consistency with other components.

Since hiddenUntilFound requires keepMounted={true} to work, using it will override keepMounted and show a warning in dev mode if both hiddenUntilFound and keepMounted={false} are specified.

[mui-bot]

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[mj12albert]

@colmtuite @vladmoroz Do you think it's an issue that 2 props are required to use hidden-until-found now?

<Collapsible.Panel hiddenUntilFound keepMounted>
  {/* content */}
</Collapsible.Panel>

[vladmoroz]

@mj12albert I think we should always mount the panel when hiddenUntilFound is used, regardless of keepMounted

[michaldudak]

If keepMounted is undefined, then yes, we can make hiddenUntilFound make the component always mounted. But when it's explicitly set to false, I'd show a warning (in dev mode only, though).

[mj12albert]

I tested this warning manually 😓

[mj12albert]

Also includes a lot of rework to the panel internals as the Select PR changed the timing of the useTransitionStatus hook and set data-entering 1 animation frame sooner than before: atomiks@cfd185a#diff-ecb73c88ec0196a8b56a56bb2b147a4d57e6f461c854c51e7a3109408336fb6c

This broke CSS transition handling, as it was using that 1 frame between the open state changing and data-entering to measure the fully expanded size of the panel contents before applying the transition (which starts when data-entering is applied)

[mj12albert]

Continued in #889

Requires #887