Add single-line IF statements

[Julio Merino]

It seems like writing one-line IF statements is a pretty common need and QuickBASIC, for example, has special syntax for them that looks like this:

IF condition THEN statement

Note the lack of END IF and the lack of colon separators in the line.

Research this a bit more before implementing to see how these really behave in at least QuickBASIC and maybe GW-BASIC.

[Julio Merino]

This was added in dc4e0cd.