[operators][2/2]feat: Add Assignment operation expr with xor assignemnt

[summer-ji-eng]

Enables ^= operator and add unit test.

[miraleung]

LGTM with nits addressed.

[miraleung]

Consider checking up-front and returning false first if:

• One of variableType or valueType is not a numeric type.
• valueType is boxed or is a floating-point type.
• valueType is a floating-point type.

[summer-ji-eng]

I could do like

    private boolean isValidXorAssignmentType(TypeNode variableType, TypeNode valueType) {

      // LHS is boolean or its boxed type, RHS should be boolean or its boxed type.
      if (variableType.equals(TypeNode.BOOLEAN)) {
        return valueType.equals(variableType);
      }
      // LHS is integer boxed type, RHS should be non-floating-point numeric types or their boxed
      // types.
      if (variableType.equals(TypeNode.INT)) {
        return TypeNode.isNumericType(valueType) && !TypeNode.isFloatingPointType(valueType);
      }

      if (!TypeNode.isNumericType(variableType) || !TypeNode.isNumericType(valueType)) {
        return false;
      }
      if (TypeNode.isFloatingPointType(variableType) || TypeNode.isFloatingPointType(valueType)) {
        return false;
      }
      if (TypeNode.isBoxedType(variableType)) {
        return false;
      }
      return true;
    }

But I don't see the point of checking up-front. Could you explain more the benefit to do so?

[miraleung]

When rearranged this way, we can now simplify it as follows:

return TypeNode.isNumericType(variableType) && TypeNode.isNumericType(valueType)
    && !TypeNode.isFloatingPointType(variableType) && !TypeNode.isFloatingPointType(valueType)
    && !TypeNode.isBoxedType(variableType);

The original benefit was to make the true-checks easier to read, but this rearrangement works too.

[summer-ji-eng]

Updated, LMK what you think on new version.