Split up OpenSCD/openscd into 2 packages

[pascalwilbrink]

In order to migrate towards OpenSCD Core more easily, we need to split up the current openscd/packages/open-scd package.

During this story, The OpenSCD monorepo will have 3 packages:
- packages/core
- packages/openscd
- packages/plugins

Core

Core will stay the same

OpenSCD

OpenSCD package will contain the OpenSCD class and all the mixins

Plugins

Plugins package will contain all the plugins and their related files (such as helpers / foundation files)

Acceptance Criteria:

• OpenSCD will still build successfully
• the plugins.json file will be inside packages/openscd
• GH Actions still work

Important

Building order (will be fixed in #1420 )