[QML] Default parameters in ES functions throws syntax error

As claimed in the documentation, currently the JS engine is supposed to be compliant with the 7th edition of ECMA-262.

However, as of the 6th edition, one should be able to set default values for function parameters:

function multiply(a, b = 1) {
  return a * b;
}

console.log(multiply(5, 2));
// expected output: 10

console.log(multiply(5));
// expected output: 5

Currently we get the following compilation error at the function declaration point:

error: Expected token `)'

This issue might be the cause for QTBUG-89604