aspl: add (inline) `if` expressions/a ternary operator

Feature idea

There is currently no clean way to conditionally assign/select values inside a single expression in ASPL, leading to a lot of boilerplate code like temporary or wrongly-typed variables. See this code, which is taken from the ASPL compiler, for an example:

var string? mainOutFile = null
// ...
if(Options:targetOs == "android"){
    mainOutFile = tempFile
}else{
    // ...
    mainOutFile = exeFile
}

Possible solutions/implementations

An if expression, analogous to if statements, or a ternary operator, which is just a different notation for the same thing.

Alternatives considered

Match expressions (see #37) are the more general solution to this problem but also unnecessary clutter for just a simple binary condition, so additional if expressions are still worth it, imo.

Additional context

No response

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