Debugger improvements

[isc-bsaviano]

• Fix the automatic termination of REST and unit test debug sessions (issue reported to the WRC)
• Prevent VS Code from showing a Create File button for files that are deployed (issue reported to the WRC)
• Don't attempt to set a breakpoint that's from a file in a different workspace folder
• Fix debugging files in a client-side virtual workspace folder
• Explicitly report failure to set a breakpoint
• Use the document index to get determine the name of a document in a local file
• Cache the contents of files fetched during a debug session