Introduce gpui2 and storybook crates (#2919)

This PR splits `crates/gpui/playground` with two new crates: `gpui2` and
`storybook`.

GPUI 2 re-exports most of `gpui`, but makes some adjustments. I want to
keep the scope focused. This isn't literally version 2.0 of GPUI or
anything. It's just a convenient way to make a create with fewer
dependencies where we can iterate quickly on aspects of GPUI's design.
Most of the focus is on improving our approach to element layout and
styling.

The `storybook` crate is pretty empty for now, but it's where I intend
to start rebuilding interfaces. I welcome anyone else to join me in
this, though until I get one interface fully built, buyer beware. You
may need to pair on it with me.