[Bug] Return Type of Methods about bignumber.js HOT 1 OPEN

After working for a while, I realized the problem was deeper than I thought. The code contains 56 hard-coded new BigNumber() calls.

```class ExtendedNumber extends BigNumber {
  bar(): this {
    return this.add(1).decimalPlaces(2);ts(2339)
  }
}

let value = new ExtendedNumber("10.1"); // value is instance of ExtendedNumber
value = value.bar(); // value is instance of BigNumber !!!!!

I tried to replace all new BigNumber() with new this.constructor(). However, in some places, this is not available, so I tracked the calling code and replaced, for example, parseNumeric(...) with parseNumeric.call(this, ...) to make this available in the parseNumeric() function.

Unfortunately, when I changed maxOrMin.call(this, ...), tests failed to the degree that I needed to investigate much more deeply, and I couldn't continue.

In the current situation, BigNumber.js seems hard to extend in a subclass. I'll be happy if you can change that, but it may need much more effort than you want to spend.

Nonetheless, this is a very good library.

Kind regards,

from bignumber.js.