Typescript support

[Psycarlo]

Description

Missing decode function in TypeScript type declarations bolt12.d.ts.

Suggested fix

Add to bolt12.d.ts:

type Section = {
  name: string;
  value: unknown;
};

type DecodedOffer = {
  offerRequest: string;
  sections: Section[];
};

declare function decode(offerRequest: string): DecodedOffer;

export {
  // ... existing exports
  decode,
  type DecodedOffer,
  type Section,
};