Preserve the text style when splitting at the end of paragraph.

At the moment, splitting at the end of the following paragraph
<p style-name="standard">hello <span style-name="bold">world|</span></p>
will create a new paragraph like below
<p style-name="standard">|<span style-name="bold"></span></p>

The cursor is moved out of the span due to empty span is not considered as
a valid cursor position in the current design. this causes newly entered
text are not formatted correctly.
Pass down the directParagraphStylingEnabled flag to DirectFormattingController.js, disable the paragraph related methods if the flag is true.