fix!: decode large non-safe numbers with no decimal as BigInt

[rvagg]

Heads up @achingbrain, this is going to make dag-json do the same thing that dag-cbor does, returns BigInt when you're outside of safe integer range. We're currently in a lossy situation with those numbers, but with this change we get to safely pull the integer out as it is in the encoded JSON.

But, it's a breaking change because previously it's all just Number.

---

BREAKING CHANGE

Because previously, all numbers just get decoded as Number, after this
change any number in the JSON that doesn't have a decimal place and is
outside of the JavaScript safe integer range will be provided as a
BigInt.

[rvagg]

picked up with https://github.com/ipld/codec-fixtures, failing tests in the JS stack:

• int--11959030306112471732
• int--9007199254740992
• int--9007199254740993
• int--9223372036854776000
• int-11959030306112471731
• int-18446744073709551615
• int-9223372036854775807

this changes fixes all of those and lets us round-trip properly through the different codecs