Make last argument to `mid$` optional

[James-Livesey]

Like other implementations of BASIC, the length argument to mid$ should be optional to get the rest of a string from the starting index, just like substring in JavaScript.

Example (this should print ello:

10 print mid$("Hello", 1)

Currently, we get an Expected another argument to function error as mid$ expects three arguments.

Documentation will need changing to reflect the argument's optionality as well.