Add native TypeScript enum support to BorshSchema serialization

[r-near]

Currently, when serializing TypeScript enums with Borsh, we need to manually convert enum values into an object format like { EnumValue: {} }. This isn't idiomatic TypeScript and creates unnecessary boilerplate.

Current usage:

enum Status {
  Pending,
  Fulfilled,
  Rejected
}

const status = Status.Pending;
const schema = BorshSchema.Enum({
  Pending: BorshSchema.Unit,
  Fulfilled: BorshSchema.Unit,
  Rejected: BorshSchema.Unit
});

// Need to manually convert to object format
const buffer = borshSerialize(schema, { Pending: {} });

Proposed change:

• Add automatic conversion of TypeScript enum values during serialization
• Allow direct serialization of enum values without manual object conversion

Desired usage:

enum Status {
  Pending,
  Fulfilled,
  Rejected
}

const status = Status.Pending;
const schema = BorshSchema.Enum({
  Pending: BorshSchema.Unit,
  Fulfilled: BorshSchema.Unit,
  Rejected: BorshSchema.Unit
});

// Just pass the enum value directly
const buffer = borshSerialize(schema, status);

Implementation requirements:

1. Detect when a value is an enum value during serialization
2. Convert it to the required object format internally
3. Support both numeric and string-based enums
4. Maintain backward compatibility with existing code

This would make the library more ergonomic for TypeScript users while maintaining the same underlying Borsh encoding format.

[hanakannzashi]

The enum type in Rust is essentially a union structure, so this expression looks quite strange in JavaScript

For enum values WITHOUT additional associated values, maybe we can directly use a union string type as a replacement. i.e.

type Status = 'Pending' | 'Fulfilled' | 'Rejected';

// This is compatible with enum
enum StatusEnum { 
  Pending = 'Pending', 
  Fulfilled = 'Fulfilled', 
  Rejected = 'Rejected' ,
}

Then the usage

const schema = BorshSchema.EnumLiteral(['Pending', 'Fulfilled', 'Rejected']);
const buffer = borshSerialize<Status>(schema, 'Pending');
const status = borshDeserialize<Status>(schema, buffer); // 'Pending'