Add Multi-Token (NEP-245) Support to `@near-js/tokens`

[denbite]

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• I have read the Contributing Guidelines on issues.

Description

The @near-js/tokens package currently supports Native and Fungible tokens. To improve interoperability and align with standards, we'd like to add support for the Multi-Token Standard (NEP-245).

This is a great opportunity for community contributors to kick in. Contributions should include tests and follow the existing coding conventions in @near-js/tokens.

Has this been requested on GitHub Discussions?

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Motivation

With the rise of NEAR Intents, developers are increasingly working with Multi-Tokens (NEP-245) and supporting this standard directly in the core @near-js/tokens package will drastically reduce boilerplate code.

API design

• Add a new MultiToken class, following the same design principles as FungibleToken and NativeToken.
• Implement view methods getBalance, getBatchedBalance
• Implement call methods transfer, transferCall, batchTransfer

Self-service

• I'd be willing to contribute and develop this feature myself.