Improve scope highlight hook

[Danielkonge]

It is not always a good idea to pick the highlight color based on the last symbol of the start/end line when using e.g. rainbow-delimiters. E.g. in Lua, I have some functions that look like this:

...
... function()
  ...
end,

The current code would pick highlights based on the "," (no highlight) and the ")" (wrong highlight, this is one level deeper than function-end).

I have had better luck picking based on the actual scope start/end, and then I have left the old last symbol on the start/end line as a fallback. This picks the color from function and end in the above example.

[lukas-reineke]

Looks good, thanks.
Just some suggestions to make the comments clearer.