Draft: feat!: itemize skills

Implements skills as items, making wide variety of easy transformations and handlings possible.

Done:

• Add skill item type.
• Skill preset configuration dialog.
  • Background skill toggle moved into the editor.
  • Pack selection searches existing packs and provides choice of packs not yet selected but ones that contain skills.

TODO:

• Implement functional skill sheet
  • Skill grouping
• Implement standard skill packs
  • Core skills
  • Background skills
  • Consolidated skills [#632]
• Swap actor sheet skill list rendering
• Ensure change system is not broken
• Ensure skill rolling and skill info retrieval works
• Ensure drag&drop to hotbar is not broken
• Add update shims and deprecation notices.
• Add migration
• Add clear warnings about skill ID conflicts.
• Add easy way to convert existing actors to current skill preset (such as a button on sheet settings tab).
  • Synchronization dialog for showing up what is being changed and having checkboxes for approving the changes.
• Add consolidated skills rules reference.

Implements #420, #578, #632, and #756

Closes #2130

Invalidates and supersedes !879 (closed)

Screenshots

Configuration editor: