Add support for custom emoji shorthands

[zamsyt]

I like to mark certain information in my notes with emojis. It would be nice if Dataview could understand them too.

This would also allow people to define new shorthands for task metadata without the plugin needing to implement them.

Examples of what it might look like:

🌐 https://example.com
🔑 "these", "are", "keywords"
💡️[[some note]]

- list item 📖️https://example.com
- list item [📖️ Perhaps A Book]

- [ ] task item 👤️Some Person

Then, in settings, I would define these as custom shorthands, by assigning a textual key to each emoji, e.g. something like:

• 🌐 - "url"
• 🔑 - "keywords"
• 💡 - "inspiration"
• 📖 - "source"
• 👤 - "person"