New paragraph layout 🚀

The previous paragraph layout algorithm had a couple of flaws:
- It always produced line break opportunities between runs although on
  the textual level there might have been none.
- It didn't handle trailing spacing correctly in some cases.
- It wouldn't have been easily adaptable to Knuth-Plass style optimal
  line breaking because it was fundamentally structured first-fit
  run-by-run.

The new paragraph layout algorithm fixes these flaws. It proceeds
roughly in the following stages:
1. Collect all text in the paragraph.
2. Compute BiDi embedding levels.
3. Shape all runs, layout all children and store the resulting items in
   a reusable (possibly even cacheable) `ParLayout`.
3. Iterate over all line breaks in the concatenated text.
4. Construct lightweight `LineLayout` objects for full lines instead of
   runs. These mostly borrow from the `ParLayout` and only reshape the
   first and last run if necessary. The design allows to use Harfbuzz's
   UNSAFE_TO_BREAK mechanism to make reshaping more efficient. The size
   of a `LineLayout` can be measured without building the line's frame.
5. Build only the selected line's frames and stack them.

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