feat: flag to toggle trailing / for arial-current

- Implement handling of situation where a non-root href with a trailing
  slash only matching exact trailing slash and not the other version,
  for example, an `href="/item/"':

  - location="/item" will not match;
  - location="/item/" will match, and thus aria-current="page" set.

- Relevant comment in test has been amended to suggest that there may be
  another solution where only the immediate parent location should have
  the flag set.