[New] Add 'fixed' option to allow position-fixed appendToBody pickers

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Motivation

We are trying to use the date-picker in a small position: fixed container with overflow: hidden. To make it escape its container using a portal, we use appendToBody={true}

This mostly works, but because the container is appended to the body in a static layout, the date picker becomes invisible once we scroll down the page.

Fix

1. Add appendToBody="fixed" as an additional option, in addition to the normal booleans. Update all PropTypes

Testing

Added new automated Mocha tests that verify that position: 'fixed' gets added when appendToBody="fixed" is set

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@ljharb Is there any chance you can review/merge this in? I'd appreciate it!