Resolve .NET Core debugging on Windows

[philliphoff]

gdziadkiewicz had discovered that I broke debugging .NET Core in Docker on Windows with the previous Alpine support commit, and was also kind enough to suggest a workaround in PR #795 (a larger change to support F# project debugging).

For simplicity, I've split that workaround into a separate PR and updated/verified it to work across Mac/Ubuntu/Windows. The changes are:

• Use a double-quoted script string to obtain the container debugger type when VS Code is running on Windows.
• Normalize the relative (host) path returned by the debugging client for the target (i.e. container's) platform.

[philliphoff]

@StephenWeatherford @EricJizbaMSFT Could either of you take a quick look at this when you get a chance (as a community PR is dependent upon it)? Thanks!