[ty] Improve union and intersection builder performance (#27167)
## Summary
The `add` operations move the `Union`/`IntersectionBuilder`. This can be
noticeable in hot mappings and normalization where the fluent API also
isn't required. This PR uses `add_in_place` in more places and also adds
such an API to `IntersectionBuilder`.
This improves performance by about 1%. I extracted this out of the
scripts PR, so please remind that when considering the regression of
this PR. I discovered this optimization because the scripts refactor PR
increased the size of `UnionBuilder`, making the move more expensive.
Since I'm out Monday/Tuesday, feel free to merge.
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Co-authored-by: Carl Meyer <carl@astral.sh>
