Expose tag settings

BBCode tag plugins can define settings, which are part of the plugin configuration and therefore technically editable for each text format.

This would allow, eg., a [code] tag that has line numbers turned on or off by default.

Unfortunately, those settings currently can't be changed without editing the site configuration directly. They should probably be editable in a modal ( Drupal 8 actually has those! → ) to avoid cluttering up the main table.

For full tag plugins, this is just a matter of adding a TagPluginInterface::settingsForm().

For entity-based tags, the form must be generated from inspecting the schema (which lives under xbbcode.tag_settings.{...}) for the appropriate types and labels.