Support saveBeforeStart setting in debugPreferences

[sgraband]

Feature Description:

Thedebug.saveBeforeStart option in the settings.json is marked as unsupported and does not have an effect. Files are always saved when starting a debugging session.

So the options needs to be added to the preference schema as well as taken into account when trying to save the files before starting a debugging session.

The expected behaviour is to have three options available:

• allEditorsInActiveGroup: Save all editors in the active group. The default value.
• nonUntitledEditorsInActiveGroup: Save all editors in the active group except untitled ones.
• none: Don't save any editors.

For more information please see the announcement blogpost or the code.