Improve webform support

• Make phone's composite elements available as webform tokens
• Consolidate webform's phone number settings into a single fieldset.
• Render field description because PhoneNumber form element expects it to be a string not a render array (originally reported in the issue #3059901)
• Make the default placeholder text translatable
• Simplify code for adding phone number properties to the form element
• Improve description of the webform phone_number element.

Closes #3036641