Support big int in approximently

[koddsson]

Added a new assertion property numeric which checks if given variable is of type Number or BigInt. Then used that new assertion to replace a is.a('number') check in the closeTo/approximately assertion.

Also replaced a Math.abs function call which doesn't support BigInt which all adds up to supporting BigInt in closeTo/approximately.

TODO

• Write more tests for numeric assertion.

Follow up

• Look for more places where we should support BigInt.

Fixes #1465

[koddsson]

@43081j; let me know what you think of 45f8825