Format lambda body like value declaration expressions

[jmackie]

As part of adding if-then-else expressions (#21) I realised there are a few different ways we want to format expressions as part of value declarations.

-- inline
five = 5;
inline_if = if true then yeh else nah;

-- multiline hanging (calls, arrays...)
hanging_call = some_function(
    arg0,
    arg1,
    arg2,
);

-- multiline newline (if-then-else, let, matches...)
multi_line_if = 
    if true then
        yeh
    else
        nah;

But this same logic also applies to the function bodies. Hence I've moved this logic to a helper function, now used by both value declaration formatting and "lambda" formatting.