Add line numbers to the ConstantDefinition tracker

[vinistock]

Motivation

Based on this suggestion.

We need the line numbers where constants were defined in order to add the original gem source locations for jump to definition. This PR saves this information in the ConstantDefinition tracker.

Implementation

Added a T::Struct to keep track of path and line number, started populating it in the tracker and used the new method in the only occurrence of files_for.

Tests

I contemplated adding a new test only for this tracker, but noticed that's not a pattern we're using in tests. Should I create a new one for this tracker?

[paracycle]

I am ok accepting this without a test since it is relatively hard to test it properly (since we would have to setup multiple Ruby files, etc) and there isn't that much logic in the tracker in the first place.