This PR removes the `em_width` and `em_advance` parameters to
`EditorSnapshot::gutter_dimensions` in favor of computing the values
inside of it.

In practice all of the callers were passing in the same values, and
there isn't a circumstance where we would want to pass in different
values.

`gutter_dimensions` has also been modified to return
`Option<GutterDimensions>` instead of `GutterDimensions` so that we can
remove some `.unwrap`s when interacting with the text system.

Release Notes:

- N/A