Enhance function expression metadata

[ekhaled]

export let subtract = function(a, b){
  return a - b;
}

export let multiply = (a, b = 1) => {
    return a * b;
}

These VariableDeclaration items contain FunctionExpression and ArrowFunctionExpression within them.

We should treat these similar to FunctionDeclaration and make parameters and return values available in the output.