Boa: Implement `BigInt` static methods

[HalidOdat]

ECMASCript feature

Implement BigInt static methods:

• [x] [BigInt.asIntN()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt/asIntN)
• [x] [BigInt.asUintN()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt/asUntN)

More information:

• ECMAScript BigInt object
• MDN BigInt object

[Tropid]

I can take this. Seems doable :D

[HalidOdat]

I can take this. Seems doable :D

Sure! Tell us if you need any help! :)

[Tropid]

Quick status update:

After I was struggling a bit, I made some progress. The official ECMA conformance test suite is a great help (https://github.com/tc39/test262/tree/master/test/built-ins/BigInt/asIntN). It seems like the functions return the correct values for valid inputs now.

The error handling is not done yet and the code has to be cleaned up some more. I'll continue working on it.

[HalidOdat]

450 should make it easier for error handling

[Tropid]

This sounds great, especially "Merged Ast BigInt to Builtin BigInt.". The three different BigInt types were really confusing me.

Merging this will be difficult, though 😅