Expressions, and *IF support

[stevesims]

RISC OS's CLI supported a *IF command, which would evaluate an expression and call a command if the expression evaluated as true. *if <expression> then <command> [else <command>]

https://www.riscosopen.org/wiki/documentation/show/*IF

As noted against comments on #54 we may look to eventually support functionality similar to OS_EvaluateExpression

(This type of command is mostly only useful once the CLI supports system variables)

[stevesims]

*if is now present in the MOS 3 alpha 1

Currently there is no expression engine - only "truthy" expressions will "pass"