[css-lists] [css-pseudo] need to define blockification of outside markers / interaction of display: inline list-item with list-style-position

Outside markers are defined to create block containers. This is as of today implemented in both Gecko and Blink at least as a computed-style-time hack:

• Gecko.
• Blink.

Right now that has a visible effect via getComputedStyle(element, "::marker").display (this test tests for it).

However:

• The spec should probably say how it is implemented given it is observable.
• That is not quite sound with display: inline list-item, because the display value may be ignored / overridden.

The last bullet point is, of course, if we agree that display: inline list-item ignores list-style-position (which Gecko does right now), but it seems a bit under-defined.

So in Gecko, I'm going to change this to be a used-value-time blockification (mostly to prevent the case where we have a block element like <details> with display: inline list-item and fail to blockify the marker). But I'm happy to adapt to whatever solution we end up with here since we're the only ones implementing inline list-item after all.

cc @Loirooriol @MatsPalmgren @jfkthame

Start with css/css-pseudo/marker-display-computed.html and the linked Gecko and Blink style-adjuster implementations, then trace the existing CSS Lists and CSS Pseudo definitions for outside markers and inline list items. Done means the specification explicitly defines blockification and the interaction with list-style-position, with the existing computed-style behavior covered by the test.