Automatically add `compat` entries according to current installed versions

[dpsanders]

compat entries should automatically be added according to the current minor version of the relevant package whenever that package is Pkg.added to an environment.

[ufechner7]

This cannot be so difficult... What is missing? Do we want that this works in the way described by dpsanders?
I think that would be very useful for beginners to avoid that they upgrade packages and brake their code because they added a package.

[ufechner7]

Is this a duplicate, or what is different: #776 ?

[IanButterworth]

Yeah. Duplicate of #776