Split up OpenSCD Core and OpenSCD Plugins

[david-monichi-bp]

Is your feature request related to a problem? Please describe.

Based on the decision made in #1599 BearingPoint is porting the split of Core and Plugins, which has been done for TransnetBW, back to the OpenSCD project.

Describe the solution you'd like
Here the over all documentation #1599, following the repository of the two components:

• Core will be remain in repository: https://github.com/openscd/open-scd
• Plugins will be moved to repository: https://github.com/openscd/oscd-official-plugins

Requirements
OpenSCD works as before

Additional information
According to the decisions made in the weeklys and documented here #1599 we are going to split the plugins from the core. This Issue is just to make it traceable, implementation will follow soon.

Test Files
OpenSCD works as before, we would ask you guys to keep an eye open to unexpected changes after beeing done with it.