Support for custom editors in the workbench

[bpasero]

To support custom editors (#77131, #12176) we need our IWorkingCopyService to become more powerful, specifically taking over responsibility of:

Work Areas:

• dirty tracking
• save/revert
• auto save
• hot exit
• structural changes (move, delete, create, copy)
• save participants (text file based only, we do not plan for an API for other extensions to modify the custom editor data)

[bpasero]

I think we settled on saving everything, even working copies from custom editors.

[DonJayamanne]

@bpasero @mjbvz Is the plan to (try) ship the custom editor in the next release?
I'd like to ensure we (Python Extension) uses this when its available.

[mjbvz]

No. It will remain as proposed api in 1.41