feat: support comment c8 ignore start/stop

[connorjclark]

I implemented the idea I shared here: bcoe/c8#271

/* c8 ignore start */
function dontMindMe() {
  // ...
}
/* c8 ignore stop */

I refactored the function that parses for these c8 ignore comments to have no side effects; I also split up the "on its own line" case to be handled separately from the "ignore next n" case. My goal was to make the function and its usage more readable: let me know if I failed in that goal.

[bcoe]

@connorjclark awesome, thanks for the contribution 👏

Sorry for the slow review, was driving across the country last week.