fix: expose CompileError interface, not class

[Rich-Harris]

We expose the CompileError interface, but we don't want to expose the class (all compile errors should come from within the compiler; it shouldn't be possible for someone to create their own).

Currently, we get this wrong — we export the class rather than the interface. This fixes it, and removes some duplication.

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Latest commit: f9fffe2

The changes in this PR will be included in the next version bump.

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[Rich-Harris]

gonna go ahead and merge this to unblock #12239, because if we don't merge that we're gonna end up with hella conflicts