Add an if-return Guard Syntax

[smores56]

We implemented a convenient reformatting of if-else statements to act as "early returns" at the start of functions in #7039.

Because of the planned introduction of a return keyword that actually performs early returns, we want to add a syntax for if-return guard statements like the following:

processData = \data ->
    if dataIsInvalid data then
        return Err InvalidData

    useData data

Once that is done, we should remove the if-else "early return" from #7039 and go back to the old formatting style for if-else.