Create Line "tree"

This will be a tree-like structure of nested `describe`, `context`, and
`it` blocks.

Right now it is made up of anything that is nested within each other,
from the top of the file down to where the cursor is.
Add JavaScript tests to File tests
Remove unnecessary File tests

Remove File tests that were testing if the File gets the lines up to
where the cursor is. That is an implementation detail, and we don’t
need to test it.

We do test that we can generate the tree of lines by indentation level,
which is what we really care about.