data-options doesn't override existing config

Problem/Motivation

The data-options introduced by #3493832: Allow custom data-options to be added to elements JavaScript options isn't working properly.
It allows adding new attributes, but not overriding existing ones.

Steps to reproduce

1. Create a new webform and add a telephone field.
2. In the field settings, activate the Enhance support for international phone numbers.
3. Go to the Advanced tab and add the following to Element custom attributes (YAML):
   

   data-options:
     hiddenInput: '{"nationalMode": "true"}'
   

By default, the nationalMode is false and adding this data-options doesn't override the settings.

Proposed resolution

Instead of options = $.extend(JSON.parse($telephone.attr('data-options')), options); change it to:
options = $.extend(true, options, JSON.parse($telephone.attr('data-options')));
It will also allow the two objects to be merged.

Remaining tasks

User interface changes

API changes

Data model changes