Bug 30059: Add JS equivalent for Koha::Patron->get_age

Bug 30059: Add JS equivalent for Koha::Patron->get_age

On bug 30055 we are going to use the REST API to display the patron
search result, we will then need to calculate patron's age client-side.
This is moved to its own bug report in case we need to reuse it
somewhere else.

Test plan:
Copy/paste the JS function in your browser's console then call it and
confirm that the result is correct
For instance:
  $get_age('2000-01-01')

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>