Default project

[davidanthoff]

Here is an idea that takes the ideas in #1095 to the next step.

We could have a default project at .julia/environments/default and julialauncher would start the actual julia process with that project active if the user didn't provide an explicit project via the command line or an environment variable.

This would have a couple of interesting implications:

• The "default" channel setting of Juliaup would be more or less unnecessary. The "default" Julia version on a user system would just be the one recorded in the "default" project manifest.
• It would be very rare to run Julia without an active project, it would essentially be either the default project or a user specified project is active. I think this would have major benefit in that users couldn't so easily add packages to the vX.Y projects that are always loaded because of stacked environments (essentially my point https://discourse.julialang.org/t/speculations-about-the-default-environment-or-a-new-draft-environment/78967/28?u=davidanthoff).
• The whole logic on how julialauncher informs users about new Julia versions would presumably be a bit different. I think it would be 1) check the Project.toml for a compat entry for julia, 2) look at the Manifest.toml and the database of existing Julia versions to decide whether there is a newer version that the user should be alerted to. This would of course also be useful just for Launch the manifest-specified Julia version v3 #1095.
• For this to happen, we would need much better tools to update manifests across Julia builds.