Add a formatSize method to NumberHelper

Let's introduce a method to format file sizes.

Examples:
* `formatSize(0)` ⇨ `0 bytes`
* `formatSize(1)` ⇨ `1 byte`
* `formatSize(1000)` ⇨ `1,000 bytes`
* `formatSize(1024)` ⇨ `1.00 KB`
* `formatSize(2000000)` ⇨ `1.91 MB`
* `formatSize(-5000)` ⇨ `-4.88 KB`
* `formatSize('hello')` ⇨ `hello`