[FEATURE] `Package` functionality

[georg-schwarz]

User Story

1. As a {user}
2. I want {to group elements into reusable packages}
3. So that {I can use the package as entry point instead of searching in the single files for the elements}

User Acceptance Criteria

• Corresponding parts of RFC 0015 are implemented
  • package functionality
• Suiting validations
• Suiting auto-complete
• Suiting code actions
• Suiting jump-to-definition behavior
• Documentation
• Examples

Examples

See RFC 0015.

Notes

Related issue: #563 to introduce the publish and use concepts of RFC 0015

Definitions of Done

• A PR has been opened and accepted
• All user acceptance criteria are met
• All tests are passing