Refactor workspaces to provide a scaffold for Workspaces UI

[krassowski]

References

Code changes

• move workspace logic and commands from apputils-extension:workspaces to dedicated workspaces-extension
  • this enables building upon these commands
  • the mime extension that .jupyter-workspace files on click was retained in apputils-extension:workspaces for now (STC); it can stay there because (a) this is not needed in workspaces-extension and (b) this way we do not remove apputils-extension:workspaces plugin (reducing friction for users who disabled it before)
• add open command
• add commands to command palette
• simplify saveAs logic by using InputDialog.getText() (which gives us pre-selection and placeholder)

TODO:

• decide if we want to have both workspaces (for now only a single token) and workspaces-extension or just the latter

User-facing changes

Backwards-incompatible changes

None