Pkg3: Environment manipulation in the language instead of with command line arguments

If environment manipulation gets moved to a command-line invocation, people are probably going to want to run it most often from shell mode or via `run()`, which isn't any better than what we have now. Otherwise many Windows users aren't going to find it at all because the usual workflow there is often not via the command line. (IDE features can help, but shouldn't be required for this to be usable.)

As a thought, what if we make this tie into https://github.com/JuliaLang/julia/issues/17997 instead? Instead of loading packages into `Main`, load them into an environment-specific namespace. Enter and leave environments via `Pkg3.enterenv("newenv")`, `Pkg3.leaveenv()` etc. Make `Pkg3.add`, `Pkg3.rm`, and `Pkg3.update` take an environment argument. If the user attempts to update or remove a loaded package from an environment, that environment needs to be reset via something similar to `workspace()`.