Show information on installed package versions

[VeryMilkyJoe]

It would be useful to be able to see the installed version for each package dependency in a cabal file.

We could either show this in form of a code lens, or on hover, but we probably want both, seeing that users might turn off code lenses.

For implementing this, we can look at the project's plan.json file and simply find the entry where name corresponds to the package's name and then show the version of this entry.

[lucalabs-de]

I'll work on this.