Move fragment slicing to .js

[Jelleas]

The final step of compare50 is render:

compare50/compare50/_renderer/_renderer.py

Line 71 in 3727b46

def render(pass_to_results, dest):

This takes in a collection of Compare50Results

compare50/compare50/_data.py

Line 297 in 3727b46

class Compare50Result:

And produces the rendered views.

However HTML does not allow for overlapping elements, and the regions (spans) of the file that match may very well overlap. To combat this problem, the RenderTask slices the code up into fragments like so:

compare50/compare50/_renderer/_renderer.py

Line 293 in 3727b46

class _FragmentSlicer:

In switching to dynamic rendering, this task needs to happen in JavaScript instead. Something like:

function create_fragments(code, spans) => fragments