feat: add css templates

[Meierschlumpf]

Describe the feature you'd like to request

In advanced options of items, categories, dynamic sections and in custom css part of board settings we add a new functionality.
Goal of that is to add css templates, that are defined somewhere centralized (for example on a repository). When you select a template you might be able to add some variable values. An example:

Template for border radius

{
  "name": "Item border radius",
  "description": "Customize the border radius of the items on your board.",
  "variables": [{
    "type": "measurement",
    "key": "borderRadius",
    "label": "Border radius",
    "defaultValue": "8px"
  }],
  "template": {
    "type": "board",
    "css": "[data-type="item"]  .grid-stack-item-content { border-radius: ${borderRadius}; }"
  }
}

Which would then result in more or less the following UI shown to the user:

Additional information

No response

[FlamebergUA]

If you are to add this - that'd be awesome, and I'd like to personally ask to add:

• background color (including opacity)
• font-size
• border color
• border radius
• something for icon size, like svg icons in weather widget
• for certain items, like bookmarks, having 'display: none;' for URLs would be also amazing

[Meierschlumpf]

The final goal would be a system where templates can be created on a separate repository by anyone and those then could be used by everyone