STL: Verify vNext visualizers

[StephanTLavavej]

As we work on the vNext binary-incompatible branch, we're going to overhaul many STL data structures, often completely breaking their visualizers. Before releasing vNext, we need to audit every visualizer in stl.natvis and add a vNext version if necessary. (We shouldn't change or remove existing visualizers, because stl.natvis supports visualizing previous versions of the STL, and the VS 2019 compatible release series will remain important for some time.)

Also tracked by Microsoft-internal VSO-210364 / AB#210364.

Visualizer note: The Visual Studio debugger "visualizes" STL data structures using a file named stl.natvis. (Example location: C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\Common7\Packages\Debugger\Visualizers\stl.natvis) This file is updated by Microsoft's STL maintainers because it's deeply connected to the representations of STL data structures, but at this time it isn't part of this GitHub repo.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

[KindDragon]

Hi. Can this check somehow be automated? I would also like to automate this check for my boost visualizers

[KindDragon]

We shouldn't change or remove existing visualizers, because stl.natvis supports visualizing previous versions of the STL, and the VS 2019 compatible release series will remain important for some time.

Maybe move them to separate file?