Add inspector formatting

[Darkhogg]

Sometimes one needs to print out Big values or objects containing them, and at least in Node the output is considerably ugly and mostly useless for a human:

> new Big('1337.6942')
Big {
  s: 1,
  e: 3,
  c: [
    1, 3, 3, 7,
    6, 9, 4, 2
  ],
  constructor: [Function: Big] {
    DP: 20,
    RM: 1,
    NE: -7,
    PE: 21,
    version: '5.2.2',
    Big: [Circular],
    default: [Circular]
  }
}

I'm adding this to my projects using Big so that output is a bit friendlier:

Big.prototype[Symbol.for('nodejs.util.inspect.custom')] = function(depth, options) {
    const name = options.stylize('Big', 'special');
    const amount = options.stylize(this.toString(), 'number');
    return `${name}(${amount})`;
};

With this, the ouput would be like this:

> new Big('1337.6942')
Big(1337.6942)

which clearly shows that the value being printed is a Big and its human friendly representation

---

Please consider adding this or something similar to it to the base code so printing Big in node is friendlier to humans.

[MikeMcl]

Yes, I did have this in bignumber.js but I took it out because the reference to Symbol caused problems in some legacy browsers. I just added a note to the README instead, and I could do that here also.

Big.prototype[Symbol.for('nodejs.util.inspect.custom')] = Big.prototype.toString;

[MikeMcl]

Added to big.mjs in v7.0.0 soon to be published.