Include init & extra files as build passthrus

[MattSturgeon]

As mentioned in #2498, it can be hard to figure out what is going on with a nixvim configuration's generated lua file(s).

nixvim-print-init can help in many scenarios, but is currently unable to inspect extra files. It also seems a little hacky that we can't just access the file directly...

Nixvim's build should include such files in its passthru attr so that it can be inspected/built separately.

Additionally, the "standalone" build should inherit the underlying build's passthru attrs.