Add knip dead-code scan to check

Wires knip into `bun run check` and removes the three exports it flagged
on the first run: `getSegmentGraphemeWidths` (no callers),
`walkPreparedLines` plus its `InternalLayoutLine` type (only the test
used it, and only as a pass-through to `walkPreparedLinesRaw`), and
`unicodeBidiDataVersion` from the generated bidi data and its generator
(Unicode version is already in the file header).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>