Add option to compile dawn yourself

[terraquad]

I wanted to make my codebase compile offline (since all the zig-gamedev libraries are already there) and now I wanted to do the same with zgpu.
Sadly there aren't any guides (afaik) on how to compile dawn yourself instead of pulling the prebuilt binaries from github.

Would you mind adding that?

[hazeycode]

You can remove the remote dependency by downloading those repos and changing your build.zig dependency URLs to local paths. Does that work for you?

As for building Dawn from source; it's not simple. There's this out of date draft PR: zig-gamedev/zig-gamedev#463