special case for guard pattern match syntax if ... end => ...

[Roger-luo]

This is related to #404, consider the following code

@match node begin
    if is_leaf(node) end => nothing
end

currently this will format it into

@match node begin
    if is_leaf(node)
    end => nothing
end

I'm wondering if we can special case this?

[domluna]

we probably could. right now the we just split it no matter what but we could add an option where we consider the margin.

[penelopeysm]

#937 provides a config option that might fix this