Use underscore separation for large integers

[mtaube]

Any number greater than 4 characters should use _ for separation.

// bad
let num = 10000000

// good
let num = 10_000_000

[jpsim]

Good idea!

[marcelofabri]

Maybe it should validate that the _ are separating blocks of 3 digits as well?

Example from the docs:

let oneMillion = 1_000_000
let justOverOneMillion = 1_000_000.000_000_1