Function RU64

RU64(): TRU64
Creates a schema for unsigned 64-bit integers using JavaScript BigInt.

Use this for large integers that exceed JavaScript's safe integer range (2^53-1).

Returns TRU64
A Relish schema for u64 values
Example
```
import { RU64, createCodec } from '@grounds/schema';

const codec = createCodec(RU64());
codec.encode(BigInt('9007199254740992')).match(
  (bytes) => console.log('U64 encoded:', bytes),
  (error) => console.error(error)
);
```
See
- RU32 for 32-bit unsigned
- RU128 for 128-bit unsigned