Emoji and CJK horizontal centering

[suragch]

After the Flutter 3.16 update, the horizontal centering is off for emoji and CJK characters (as noted here):

This is not the case when setting style: TextStyle(height: 1) though:

The solution is probably in MongolParagraph and related to assumptions about the line width.

[suragch]

This would also be a good time to fix #10.