Add 'singleThread' argument to pause requests

[HampusAdolfsson]

Most execution requests support a 'singleThread' argument that specifices that the action should not affect threads other than the one specified by the 'threadId' argument. However, pause requests do not have this argument. Is there a reason for this?

I interpret the specification to say that pause requests should only pause one thread, but it seems equally reasonable to me that a client might want to pause all threads with a pause request.

[HampusAdolfsson]

Related to #214

[int19h]

Note that any debug adapter compatible with VS has to implement the "pause" request as pausing all threads, because VSDebugAdapterHost simply does not support anything else: "Visual Studio requires that all threads enter and leave break mode at the same time". But if DAP spec is to be interpreted in a sense that "pause" request should only pause a single thread, then no adapter that does it is conformant; at the minimum, it would apply to the Python debug adapter, although I suspect the actual list is longer (this is the same problem that also applied to #214). This needs to be clarified in the DAP spec; ideally, in the same way as it was done for "continue" etc for consistency.