feature request Office-Addin-Mock support CustomFunctions.Error

In an excel custom functions add in, I have to add a CustomFunctions.Error function to the global scope before running tests. I am using the following code (which I am not suggesting is sufficient to cover the use cases for the broad range of users) which gives a brief outline of another module which could be useful if added to the Office-Addin-Mock package:

enum ErrorCode {
  invalidValue = "#VALUE!",
  notAvailable = "#N/A",
  divisionByZero = "#DIV/0!",
  invalidNumber = "#NUM!",
  nullReference = "#NULL!",
  invalidName = "#NAME?",
  invalidReference = "#REF!",
}

class CustomFunctionError extends Error {
  constructor(readonly code: ErrorCode, message?: string) {
    super(message);
  }
}

const CustomFunctions = {
  Error: CustomFunctionError,
  ErrorCode: ErrorCode,
  associate: () => undefined,
};

and it is used something like (mocha/chai):

describe("myFunction", function () {
  global.CustomFunctions = CustomFunctions;
  it("throws an error when the string argument is empty", function () {
    expect(() => myFunction("", [[["a"]]])).to.throw(/require/i);
'''

Start by locating the Office-Addin-Mock package and its existing global Office API mocks. Check how similar APIs are represented and add support for CustomFunctions.Error, ErrorCode, and associate as shown in the issue. Done means an add-in test can assign the mock to global.CustomFunctions and exercise error handling without defining it manually.