This makes it possible to have multiple Dockerfiles, each with their own
`.dockerignore`. Previously any docker builds would always include
anything inside `.dockerignore`. I believe this feature may require
`export DOCKER_BUILDKIT=1` but we use that in CI already.
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
