Show information on installed package versions

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.