Make a cross platform proto project

[johnynek]

previously, the proto was only built in the cli project, which is JVM only.

This moves the .proto into a proto project, which is now cross platform. This allows moving converting to and from proto to be in core.

This sets us up to finally make a node build of the compiler tool. The motivation here is:

1. if we want to move cats-effect into core Use cats-effect in core #1185, we need to fully exercise the entire system running with the things we want to do (and a node version of the compiler has long been on the todo list to keep us honest about scalajs working).
2. a full js version of the compiler potentially opens the door to making a vscode language plugin, which needs to be in typescript as I understand, to fully be a typescript/js package, no dependency on jvm or native code. This would definitely simplify distribution.
3. once we have the nodejs compiler, we can continue to refactor the main cli app such that the cross platform bits are abstracted out and possibly even moved into core using the .jvm/.js directories.