Gitpod theme as builtInExtensions

[jeanp413]

Download Gitpod theme as builtInExtensions during build

[andreafalzetti]

@jeanp413 code LGTM - is there a way to test?

[jeanp413]

ah yeah this was related to gitpod-io/gitpod#10079 but Huiwen already approved 😄, I'll just merge it then.
In any case you could test from sources and see the gitpod theme is still there, you can just wait for the gitpod task to finish or cancel them and run them manually yarn watch to compile, ./scripts/code-server --without-connection-token to launch the server if you see the logs it will say it's downloading the gitpod-theme extension