fixes #341 - introduced debounce functionality into formatValueToModel

* Updated to fecha 2.3.2

* Moved the majority of logic from computed.value.set() to new `updateModelValue(newValue, oldValue)` method to allow other components to use similar techniques.
If `formatValueToModel` returns a function, then the function is called without setting the model value.  It is the functions responsibility to call `this.updateModelValue(newValue, oldValue)`.
Otherwise, calls `this.updateModelValue(newValue, oldValue)` to execute the original logic.

* If the input type is date, datetime, or datetime-local then it creates a debounced function for setting the value in fieldInput's mounted hook and then returns a function that calls this debounced function from `formatValueToModel`

* Reduced the repeated code for date formatting by adding a DATETIME_FORMATS constant, and using that to
get the "format" for fecha.parse()