Add support for empty state illustrations

[elycheea]

The empty state includes several pre-built illustrations:

• error (alert)
• no data (box)
• no tags (tag)
• not found (magnifying glass)
• notifications (speech bubble)
• unauthorized (lock)

Acceptance criteria

• Provide prebuilt illustrations via kind prop
  • This should be optional — by default empty state should not include illustration
  • Do we need illustration description? If illustrations are strictly decorative, we shouldn't need to separately manage illustrationDescription anymore)
  • Add support for illustrationTheme with options light or dark¹
• Add support for custom illustration — if custom illustrations are provided, this should always take precedence
• Add tests

Footnotes

1. Carbon Web Components parity has considerations for adding support for theme — need to raise this in discussions ↩