refactor: simplify

[bartveneman]

There's some heuristics that we might be able to use to simplify this whole thing:

• ranges are always reported on entire blocks (but should check this statement for correctness)
• atrules are also reported, but without the @media (at-sign + atrule name)

Simplify:

• Take all the reported ranges
• Create chunks for each range of covered and not-covered code ({ start_offset, end_offset, is_covered, css })
• Adjust ranges with atrules so they include the @media prelude
• Prettify each 'block' individually
• Adjust the ranges after prettification (we know each start/end per chunk)
• Done?

Benefits:

• Determining line coverage becomes much simpler since for each chunk we know how many lines it has
• Prettification becomes much simpler because we don't need to track tokens between before/after