Externalize OpenSCD plugins to extra repository 'oscd-official-plugins'

[david-monichi-bp]

Is your feature request related to a problem? Please describe.
Restructuring of source code organisation to simplify the plugins development

Describe the solution you'd like
Move all plugins which resides in 'https://github.com/openscd/open-scd' to repository 'https://github.com/openscd/oscd-official-plugins' and integrate this plugins within the OpenSCD build as git submodules.

Requirements
Just to get notified if plugins changes have been made so that we can remerge the last state.

Suggestions for a specific user interaction
none

Additional information
See: #1599

Test Files
System behaves the same way as it did before, no functional changes will be made.