In this exercise what I'd like you to do is:
1. Under phoneValidator, define a new custom cross-field validator called dateValidator.
-- Get both startDate and endDate from the passed in control.
-- Return an error called invalidDates if startDate > endDate
-- Otherwise, return null
2. Add dateValidator to the dateRange FormGroup
Exercise Link: https://stackblitz.com/fork/rxngforms-validation-ex4