Clarify separator format in path

Currently it is not clear what path separator should be used by a client.

For example, nvim-dap currently defaults to using /, but with adapters like netcoredbg running on windows it will expect \ in source.path. The mismatch causes breakpoints to not be set correctly.

The client could set the path separator based on the system its running on, but that breaks down if client and debugger are running on different machines for remote debugging.

One option could be to mandate in the spec that debug adapters must be able to recognize / separators.
Given that FAT16,etc. and NTFS forbids / in filenames (https://en.wikipedia.org/wiki/Filename#Comparison_of_filename_limitations), there would be no ambiguity - in that it can never be a part of the filename.

But I'm not sure if this won't also break down if there are 3 systems involved: client, debug-adapter, debugger and if the debug-adapter runs on a different system than the adapter.

Seems to be somewhat related to https://github.com/microsoft/debug-adapter-protocol/issues/182

Start by reading this issue and the related debug-adapter-protocol issue #182, then compare how clients, debug adapters, and debuggers handle path separators across local and remote setups. Done means the protocol specification states an unambiguous separator rule that covers the systems involved.