Add TypeScript typings for addIconParts

[lhapaipai]

Hi!

Small addition : addIconParts was missing its TypeScript declaration in index.d.ts.

This PR adds:

• the IconPartsFunction type describing the callback signature (iconParts, metadata, colors)
• the declaration for addIconParts itself
• its export in the default object type
• a missing text field on DrawText spotted along the way

Nothing groundbreaking, just making sure TypeScript users get proper autocompletion and type checking when using addIconParts.