Copy code from core's Plugging mixin into packages/open-scd/open-scd.ts

[juancho0202]

As an OpenSCD developer I would like to be able to use the plugging loading functionality present in the Plugging mixin from OpenSCD Core.

Tasks:

• Copying code from Pluggin mixin in core into open-scd.ts
• Alter the code to include an attribute that indicates if a menu plugin is "top", "middle" or "bottom"
• Create a foundation file to host the Plugin interface code
• Remove Plugging mixin in core
• Add tests where a plugin is passed down as an object in the "plugins" prop and it's loaded in open-scd
• Add tests for all menu plugin positions and make sure they are rendered in the correct place

Acceptance Criteria:

1. It doesn't break current functionality and loads existing plugins successfully
2. User is able to add plugins by passing down a plugins object to the open-scd tag