Use relative path for error messages.

[gdotdesign]

This PR changes the paths of the snippets in error messages to be relative to the project root - the directory where mint.json file is. This makes it easier to find since we don't need to read the whole path.

From this:

   ┌ /path/to/project/source/Main.mint:3:7
   ├──────────────────────────────────────

To this:

   ┌ source/Main.mint:3:7
   ├──────────────────────────────────────

---

The only relevant change is in src/errorable.cr, the other ones are just snapshot updates for tests.

[Sija]

I can't find the changed code in question because of the massive diff from the snapshots. Could you please split the commit in two - one with the change itself and one with the snapshot updates?

[gdotdesign]

Pretty small diff: value.file.path -> value.file.relative_path