[stdlib] Grapheme breaking: Refactor to simplify logic

- Split forward and backward direction into separate code paths.
  This makes the code more readable and paves the way for future
  improvements. (E.g., switching to a linear-time algorithm for
  breaking backwards.)
- `Substring.index(after:)` now uses the same grapheme breaking paths
  as `String.index(after:)`.
- The cached stride value in string indices is now well-defined even
  on indices that aren’t character-aligned.

Author: lorentey