build: Use a tuple for pch data

This really isn't a list because it's not homogenous data, it's really
`tuple[str, str | None] | None`, but we're using list length to decide
what to do with it, and that makes for strict null issues, as an
accurate annotation would be `list[str | None]`, which would require a
lot of `is not None` checking. By using a tuple we don't need to keep
checking length, which is more expensive than null checking.

To ensure correctness I annotated some things in the VS backend

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