Provide config schema

Problem/Motivation

The module is not providing a config schema which can lead to incorrect situations for config. After talking this through with Grimreaper on Slack we came to the conclusion the next code should be added:

ui_icons.icon_pack_options.calimals:
  type: mapping
  label: 'Calimals settings'
  constraints:
    FullyValidatable: ~
  mapping:
    size:
      type: integer
      label: 'Size'

Steps to reproduce

Install the module.
Use the icon in any place you want
Check the exported config or with config inspector that will give an error.

Proposed resolution

Add the proposed code.

Remaining tasks

See above

User interface changes

None

API changes

None

Data model changes

None