Can't use bigint literals or do arithmetic with es5 target

[austinabell]

Currently, the CLI statically targets es5 for compiling contracts, but this makes certain things within the SDK hard to do like performing arithmetic on the values returned from the API (I don't know if another target fixes this) or using bigint literals (like 100n to signify a big int).

I don't even know what the recommended way to perform a simple action like:

near.prepaidGas() - 5000000

The closest thing I could find would be:

Number(near.prepaidGas()) - 5000000

but this is absolutely horrible because on overflow it just cuts off bits and doesn't error. Casting the BigInt to any or casting the right operand to BigInt seems to throw an error

[ailisp]

It seems that we should use es2020 for compiling target, as bigint is in es2020 and quickjs support that. I'll check typescript decorator still works under that setting.