Improved number literals

I implemented highlights for underscore separators and exponential numbers.

```swift
let separatedDecimalInteger = 123_456_789
let separatedBinaryInteger = 0b10_001
let separatedOctalInteger = 0o21_17
let separatedHexadecimalInteger = 0x1a_f1
let exponential = 1.21875e1
let exponential2 = 1.21875e+1
```