Change marker-to-data logic to improve conversion inside document fragment

[scofalik]

📝 Provide a description of the improvement

Marker-to-data conversion converts marker boundaries to either an element or and attribute on an element. The decision is based on whether text is allowed at given place. So, for example, we have:

<p>Foo<comment-start name="..."></comment-start>bar</p><img data-comment-end-after="..." ... />

The problem is in that document fragment allows everything inside, to enable both conversion of bigger structures with multiple elements on top-level as well as simply a fragment of a text.

However, since document fragment allows text, markers are always converted to elements. This is problematic for two reasons:

1. For the same model, the output is simply different than what editor.getData() returns, which may be problematic.
2. There is at least one specific problem: the marker element is placed inside <ul> but upon upcast, all non-allowed elements are cleared from <ul>. (BTW this mechanism most probably is not needed and I will create additional issue for that).