underlineRgb method Null safety

Chalk underlineRgb(
  1. num red,
  2. num green,
  3. num blue
)

Create Chalk with an underline of the the specified RGB color WARNING: on some consoles without support for this, such as Android Studio, using this will prevent ALL styles of the Chalk from appearing

Implementation

Chalk underlineRgb(num red, num green, num blue) =>
    _makeUnderlineChalk(red, green, blue);