chore: align warning and error objects, add frame property

[dummdidumm]

This aligns warning and error objects to contain the same properties and have their toString methods return the same shape. It also adds back the frame property that got lost in the Svelte 4->5 transition. The only difference to Svelte 4 now is a slightly adjusted toString property (which is consistent between warnings and errors now) and a position property that contains a tuple of start/end offsets instead of a pos property only containing the start offset

closes #12151

FYI @Conduitry - is this what you had in mind?

If we wanted we could consolidate this further by giving both a common base class which they then abstract from. Should we do that? Edit: Implemented in alternative PR #12326 - choose one.

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[dominikg]

great!

[Conduitry]

Yup! This is what I had in mind. Either this or #12326 would satisfy my desire here.