Recognize punctuation before emphasis even when variation selector gets in between them #798

According to https://www.unicode.org/Public/16.0.0/ucd/StandardizedVariants.txt, U+3002 IDEOGRAPHIC FULL STOP (。) can select its form according to a following variation selector.

文。__句子。__

正體字。︁_Traditional._

正體字。︁__Hong Kong and Taiwan.__

简体字 / 新字体。︀_Simplified._

简体字 / 新字体。︀__Mainland China or Japan.__

The first full stop in the 2nd and 3rd paragraphs is U+3002 U+FE01 (TW/HK form), and those in the 4th and last ones is U+3002 U+FE00 (CN/JP form).
The both full stops in the first paragraph is just U+3002.

The current CommonMark does not recognize the emphaiss in the middle and last paragraphs.

This can be fixed together with #650.