Add numeric separators

[vieira]

Add syntax highlighting to numbers and floats with numeric separator as defined in tc39/proposal-numeric-separator.

let fee = 123_00;       // $123 (12300 cents, apparently)
let fee = 12_300;       // $12,300 (woah, that fee!)
let amount = 12345_00;  // 12,345 (1234500 cents, apparently)
let amount = 123_4500;  // 123.45 (4-fixed financial)
let amount = 1_234_500; // 1,234,500

let millionth  = 0.000_001 // 1 millionth

let nibbles = 0b1010_0001_1000_0101;

let message = 0xA0_B0_C0;