@ethersproject/bignumber is strange on invalid input

[ChALkeR]

There appears to be a regex, but it doesn't actually check anything as it's not anchored.

> require('@ethersproject/bignumber').BigNumber.from('0-0x1 whatever')
BigNumber { _hex: '0x0-0x1 whatever', _isBigNumber: true }

[ricmoo]

This should be fixed in 5.0.5.

Try it out and let me know. :)

I've also added a bunch of additional test cases for both valid and invalid strings.

[ricmoo]

Closing this now. If you have any problems, please re-open it though.

Thanks! :)