Allow to pass an optional background color argument to `cls`

[agurtovoy]

This would be a pure shorthand for paper <color>: cls, e.g.:

10 cls 0  ' clear the screen, setting the background to black

Motivation

• paper <color>: cls is a common use case
• Makes it easy to experiment with colors when just starting out
• Creates a natural path for learning about paper and statement sequencing

Prior art

• TRS-80 Color BASIC, for exampleCLS 3.