feat: io-ts-bignumber package

[SHaTRO]

Adding base-10 BigNumber codecs using the "bignumber.js" package.

[EricCrosson]

Hi @SHaTRO, thank you for this PR! I can see you've put a high degree of thought and detail into this implementation. I think this is a useful package.

My only concern is the placement of io-ts-bignumber inside numbers-ts. Now that I think harder about the scope of this repository, I quite like that it includes only JavaScript-native number types (bigint, number).

My concern with including a third-party number type that I'm not personally familiar with is I suspect I would flounder when it comes to maintenance. I'm not familiar with the release cycle of bignumber.js, or how well it adheres to semantic versioning. I have the strong impression that I would not be the best maintainer of io-ts-bignumber in terms of stability provided to its users.