Add serialized versions of themes (#6885)

This PR adds serialized versions of each of the themes that currently
ship with Zed.

In a future PR we'll be looking to make these the canonical
representations of the built-in themes.

Note that we're intentionally repurposing the `theme_importer` to do
this, so that crate is a bit rough-and-ready at the moment.

Release Notes:

- N/A