perf(turbopack): Implement `ShrinkToFit` for `AutoMap` (#79218)
### What?
Implement `ShrinkToFit` for `AutoMap`, which can be used in a `Vc`
### Why?
For consistency and to reduce max RSS
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Co-authored-by: Niklas Mischkulnig <4586894+mischnic@users.noreply.github.com>
