Ensure debugger directory exists

[philliphoff]

Ensure the debugger directory exists prior to starting the container (or, rather, before any possible use of the directory), to prevent it potentially being created with permissions different than those used by VS Code (to actually acquire the debugger).

This resolves #897.

[PrashanthCorp]

I have larger questions about the use of a fileSystemHandler - all the functionality I could see in the codebase could be done with fs-extra's functionality. I don't think that should block this PR, but I'd like some context on that soon.

[philliphoff]

@PrashanthCorp

I have larger questions about the use of a fileSystemHandler - all the functionality I could see in the codebase could be done with fs-extra's functionality. I don't think that should block this PR, but I'd like some context on that soon.

The FileSystemProvider serves several purposes, as a shield from the external library actually used (i.e. fs-extra) which allows it to be easily swapped with another if needed in the future, as an adaptor for when that library doesn't exactly provide the desired API (e.g. separate file/dir exist functions), as a convenient unit of dependency injection, and as a point for mocking in unit tests.

Could fs-extra have been used directly, yes. But consider the (currently) pending PR (#903) to replace opn, where a number of places in the code had to be touched due to the change. Had the openExternal() function existed as an abstraction layer for opn in the beginning, the change likely would have been much smaller.