add declarations for net.isIP*

I would lean towards `any` since the doc seems to be unclear if the input must be a string, and `any` would more closely match the signature of other is* functions.

Also, it's not mentioned in the documentation, but it may (or may not) be worth noting the function does have the behaivor of casting the input argument to a string.
In other words this prints true, even though the argument is not a string:

```javascript
var net = require('net')
var x = {
  toString: function() {
    return '127.0.0.1'
  }
}

console.log(net.isIPv4(x)) // true

```