Bump Pkg3

[KristofferC]

• update registries when using add, it is pretty fast to update the registry and people seem to get confused when they add packages and not get the latest version of things.
• add a precompile command to precompile all dependencies that need it in the project, so it is possible to do something like pkg> update; precompile to update and then precompile everything.
• (hopefully) allow Pkg3 to act as a drop-in for Pkg2 when it comes to the CI scripts commonly used. This should simplify the transition when we rename Pkg3 -> Pkg. When this is merged I will have to test that this works well on package.