Feature request: /vcsSplitOnEveryAssert

[robin-aws]

As part of the epic on verification stability (dafny-lang/dafny#1582), I've found that the implementation of the {:split_here} and {:focus} directives can easily be reused to isolate every assertion in its own split, just as if {:split_here} was present on every assertion. I have a POC implementation that has proven useful as a debugging tool, to identify which asserts in a procedure were by far the most expensive to verify. Focusing on improving those assertions then greatly improved the stability of the procedure.

The equivalent {:vcs_split_on_every_assert} attribute would also be useful as a blunt instrument for improving the stability of a procedure: although it would increase the total verification time quite a bit because of the overhead, it would also greatly improve the stability of the verification.