Enable unordered list marker styles

Explicitly set list marker styles are ignored. Add styles for the following:

• square
• circle
• disc
• none (bullet is invisible)
• no-bullet (bullet is invisible; align text to middle of bullet placement)
• unstyled (no indentation or bullet)

Using the styles from the Asciidoctor stylesheet will probably work. And example for testing can be found in https://gitlab.com/antora/demo/demo-component-a/blob/master/modules/ROOT/pages/lists/unordered-list.adoc