[Feature Request] Add JS_ToBigUint64() function

[TooTallNate]

There already exists:

• JS_ToBigInt64()
• JS_NewBigUint64()
• JS_NewBigInt64()

Notably missing from this list is JS_ToBigUint64(), which means I have to cast my uint64_t * value to int64_t * when using the JS_ToBigInt64() function. Would be nice to not need to do the casting part, and makes sense to add it for API symmetry IMO.

[saghul]

Makes sense!

[bnoordhuis]

For anyone wanting to work on this: it should be as easy as replacing bf_get_int64 with bf_get_uint64

[saghul]

We don't assign issues, feel free to send a PR though!