Add smart select provider api

[jrieken]

We have expand/shrink selection but it works with a heuristics which has its flaws. Instead, that should be the default but we should also invite extensions authors and language brains to tackle this. Given a syntax tree it should be easy to provide the data we need.

[jrieken]

this will help with #4795

[jrieken]

To make smart select behave consistent we will likely get ast-aware- and default-provider to work hand in hand. #64004 lists sample for default selection ranges. Given that we should merge selection ranges from different providers and I think it's reasonable to do that along containment but not overlap.

[jrieken]

current proposal: https://github.com/Microsoft/vscode/blob/2baa14fcf20acfd634146027d65c55240fc75e8a/src/vs/vscode.proposed.d.ts#L25-L31