Test: More ways to get into multi root workspace

[bpasero]

Complexity: 3

• Windows @chrisdias
• macOS @aeschli
• Linux @Tyriar

We make it easier to enter a multi root workspace from various uses where multiple folders are specified.

Via Command Line:

• Verify the new command line flag --add can be used to add folders to an active window (requires a window to be opened)
  • Verify for an empty workspace, a single folder workspace and a multi root workspace
• Verify that multiple folders specified from the command line open into a single window as multi root workspace

Via Native UX

• Verify that multiple folders picked from File > Open open into a single window as multi root workspace
• macOS: verify that dropping multiple folders on the dock icon open into a single window as multi root workspace

Via Drag & Drop

• Verify that dropping 1-N folders into the files explorer allows to add the folders to the workspace
  • for a single folder workspace this will prompt
  • currently not supported on an empty workspace yet
• Verify that dropping 1 folder into the editor area opens a window with that folder
• Verify that dropping 2-N folders into the editor area opens a window with all folders as workspace