fix!: address off by one error processing branches

[bcoe]

Due to an off by one error when determining blocks in v8, the first character of a block range is sometimes the \n from the prior line:

In the example:

function fn() {
  return true;
  /* c8 ignore next */
  console.log('never runs');
}

fn();

One of the blocks is characters 30 - 83, or:

\n  /* c8 ignore next */\n  console.log('never runs');\n

It should not be possible for a conditional block to begin with a single character, so I believe we can address this problem by making the logic that calculates branches exclusive vs., inclusive, when it overlaps with the prior line.

Refs: bcoe/c8#254