Add a tag schema type

Problem/Motivation

Part of #3175327: [meta] Integrate with UI Patterns module.

Currently in Bulma Components we use the component_string schema type for a tag field:

  tag:
    label: 'The HTML tag to use for the wrapper element'
    nullable: true
    type: component_string
    default: 'div'

Per integration with UI Patterns Settings, a tag would be considered a setting rather than a field.

Steps to reproduce

Provide a tag type based on component_string.

Default its UI type (exact key TBD) to setting.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes