Implement `Number.prototype.toExponential()`

[HalidOdat]

Currently we only have the Number.prototype.toString() implementation, we should also implement Number.prototype.toExponential().

This should maybe be feature gated.

[HalidOdat]

If no one is working on this, I'll take this issue :)

Seems like our toExponential() implementation in boa is causing the last test262 test of the Number suite to fail: https://github.com/tc39/test262/blob/main/test/built-ins/Number/prototype/toExponential/return-values.js