Improve resistor-color-duo: detect string concat #55

Describe the improvement:

When the following main method is provided, the analyzer bails early:

export const value = arr => {
  return +(COLORS.indexOf(arr[0]) + "" + COLORS.indexOf(arr[1]));
};

Instead, it should be caught by hasDigitsString, and produce the same comment.

Which exercise

resistor-color-duo

Additional context

The ResistorColorDuoSolution has a method that checks if an exercise has a constructed string. It currently only detects a string template literal that looks like ${a}${b}, but we want to also capture the following:

`${a}` + `${b}`
a + "" + b
a + '' + b
""  + a + b
''  + a + b
a.toString() + b
a.toString() + b.toString()
a.toString().concat(b)