Max safe integer is treated as unsafe

[Gudahtt]

Describe the bug
There is an off-by-one error in BigNumber.from's number parsing. The code responsible for rejecting unsafe integers will also reject the maximum safe integer (and the minimum safe integer). The maximum safe integer is, by definition, safe.

Reproduction steps

import { BigNumber } from '@ethersproject/bignumber';

BigNumber.from(Number.MAX_SAFE_INTEGER); // this will throw

[Gudahtt]

I encountered this when migrating from ethereumjs-abi to ethers. I found that ethereumjs-abi allowed me to encode this number, but ethers did not. I was hoping that, with this bug fixed, the breaking changes involved in this migration would be slightly reduced.

[ricmoo]

This isn't a bug and was intentional in v5.

It likely won't be changed in v5, but v6 will expand the range to the full mantissa width, relying on the next value (a non-zero-exponents) as the padding to ensure safety..