Test evaluatable expression provider API

[weinand]

Test for #89084:

• anyOS: @connor4312
• anyOS: @isidorn

Complexity: 3

In this milestone we've introduced new extension API to better control the behavior of the debug hover.

An EvaluatableExpressionProvider can be registered for a language to provide an expression under the mouse that the VS Code debugger will evaluate against an active debug session.

Mock debug has a sample implementation.

Verify that the new extension API works as specified in the documentation (comments in vscode.d.ts) either by modifying Mock Debug or by creating a standalone EvaluatableExpressionProvider for any language.