Convert tags into plugins.

Literally turning *every* tag into a full-featured plugin class would be a large overhead, particularly for tags that only consist of a simple template.

Ideally, modules could create a tag plugin the same way a filter plugin is created, but they could also simply define a tag processor in a config file of a form like the following:

xbbcode_processor.{module}_b:
  template: '<strong>{content}</strong>'
xbbcode_processor.{module}_img:
  selfclosing: 1
  template: '<img src="{option}" />'

Which would be equivalent in all practical respects to creating two annotated plugin classes that implement the appropriate ::getTemplate() or ::render() functions, with the added benefit that if we switch to Twig, a statically defined template can be cached and precompiled.

If processors are stored in config, then they can also be created by a web interface, which neatly allows custom tags. ... maybe processors are entities? Something like single-filter text formats?

In that case, maybe defining tags in config or extending the plugin base class shouldn't be interchangeable. Maybe a tag processor should EITHER contain a template OR refer to a plugin class. Like

xbbcode_processor.{module}_b:
  label: ''
  sample:
    content: 'Bold text'
  template: '<strong>{content}</strong>'
xbbcode_processor.{module}_img:
  selfclosing: 1
  plugin: '\Drupal\{module}\Plugin\XBBCode\MyXBBCodePlugin'

The association between tag name and processor (an N:N relationship) is global and configurable. A module can define a default association:

xbbcode_tags.{module}:
  -
    name: 'b'
    processor: {module}_b
  -
    name: 'b'
    processor: {module}_b2

(Modules should be able to provide multiple processors for the same tag, and multiple tag names for the same processor, which is why this is an unkeyed sequence.)

Ooh, thanks for the tip! I haven't really dealt with the Plugin API yet and didn't know how to dynamically provide new plugins.

Done.

(In 8.x-3.x)