fix: change initial position style based on positionFixed prop

[zkuzmic]

I have a scenario where I am autofocusing an element appearing inside of a popper, and that is causing the page to scroll to the top when initially opened. A similar issue can be found here:

mui/material-ui#16740

Their solution was to add position: fixed to the initial styles on the popper, but this library doesn't offer a way to do that. This PR makes it so that initial position style will either be absolute or fixed based on the positionFixed prop provided to the Popper component.

[zkuzmic]

Quoting a comment from @atomiks on floating-ui/floating-ui#1009

The user should always call .focus() after the popper was positioned, never before. They can also set visibility themselves too.

That makes total sense to me, although there doesn't appear to be a way to do that with the current react-popper api. Or am I missing something?

[zkuzmic]

@FezVrasta any suggestions for this PR or anything I can do to help get this merged?