qmlformat should not force multiline with arrow functions

Hi,

qmlformat does this :

a = () => updateDB(params)

//becomes

a = () => {
    return updateDB(params);
};

Arrow functions are cool to write small inline code so adding the braces for small expression adds two more line of code (total 3 instead of one) and is less readable than a oneliner.  As a consequence  `return` is added even for oneline which is useless.