Add layout="size" for size-only layout animations

[bhollis]

Implements the "size" value for layout, for symmetry with "position" and to allow for some layout-animation-driven components that grow and shrink but don't animate position as they are moved around in the document flow.

Fixes #1032.

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Sandbox	Source
Framer Motion: Simple animation	Configuration
App Store UI using React and Framer Motion	Configuration
Framer Motion: Reorder animation	Configuration
Framer Motion: growing item positionTransition issue	Configuration
Framer Motion: Image lightbox	Configuration

[mattgperry]

Hey @bhollis this looks great, thanks for the PR. There's a lot of movement with the layout animation stuff so there'll be a bit more of a delay merging this but it looks really good, great test coverage too.

[bhollis]

Awesome, take your time, there's no pressing need. Just excited to contribute to this great project!