Support virtual filesystems Fixes #154

[marcellourbani]

I basically added 2 features, logically distinct who boil down to the same concept of custom URL schema support in location URLs:

• support for virtual filesystems (i.e. "adt:a4s/foo","js-viz-download:/foo/bar" )
• support for command URLs (i.e. "command:myextension.mycommand?par1=foo&par2=bar")

The 1st is for any virtual filesystem, like i.e. ssh fs or Abap FS, current implementation tested with explicit schemas in location URLs, implicit ones might work as is or require replacing path and fs functions in getCandidateDiskPaths with vscode awere alternatives

The 2nd is for my specific use case: resolving URLs is expensive, so by encoding vscode commands as URLs I can do that only when the user clicks on them. It's a standard vscode feature, not tied to my extension