Support DAP variable paging

[roblourens]

DAP has some built-in mechanism for paging. Variables return their child count in indexedVariables and namedVariables. And the variables request includes a start and count. This lets vscode implement a paging UI that looks like this for js

It looks like debugpy has implemented its own similar mechanism, but then it doesn't let me explore the full array

Is it possible to implement this in debugpy using the properties for paging?

[int19h]

IIRC we did it that way because paging wasn't consistently used by the clients. If this works in both VS and VSCode now, we should just move to that by default. If not, it'll probably need to be under a feature flag.