Rename jetbrains.mps.editor.diagram to jetbrains.mps.editor._diagram

I will create a new language based on the language in https://github.com/slisson/mps-diagram-editor.git
Create new language jetbrains.mps.editor.diagram
Copied over most of the mbeddr diagram language
Renamed concepts. Introduced interfaces.
generated sources and classes removed from git
Delete deprecated concepts