Cache the resulting `TextRun` of a box tree layout in text nodes. This
allows reusing shaping results between box tree layouts. Currently,
these shaping results were stored in a cache in the `Font`, but this
avoids having to hash the entire string. In addition to avoiding the
performance cost of that hash, this opens up the path to two future
changes:
1. Only caching short strings: This further reduces the performance
   cost of the shaping cache and matches what Blink does.
2. Moving line breaking out of shaping: This should reduce the memory
   usage of unbreakable shaped text slices and also allow for more
   easily implementing break-anywhere type CSS features.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Simon Sapin <simon@igalia.com>