Add different counter-style for ordered lists based on their depth

Summary:
Draft.js have only 1 type of counter-style for ordered lists - the numeric.
Current Draft.js:
https://pxl.cl/d7zZ

Quip have a 3 different counter-styles, based on the depth:
https://pxl.cl/d7x6

This can be added by implementing the second argument to the counter function:
 content: counter(ol1) ". ";
to:
  content: counter(ol1,lower-alpha) ". ";

For more info: https://css-tricks.com/numbering-in-style/
Browser support is excellent: https://developer.mozilla.org/en-US/docs/Web/CSS/list-style-type#Browser_compatibility (for Basic support)

Reviewed By: flarnie

Differential Revision: D7840010

fbshipit-source-id: 6655566baafd9c03468c1b1b7c83bdb5ecf1ada6

src/component/utils/DraftStyleDefault.css

@@ -151,12 +151,12 @@
 }
 
 .public/DraftStyleDefault/orderedListItem.public/DraftStyleDefault/depth1:before {
-  content: counter(ol1) ". ";
+  content: counter(ol1,lower-alpha) ". ";
   counter-increment: ol1;
 }
 
 .public/DraftStyleDefault/orderedListItem.public/DraftStyleDefault/depth2:before {
-  content: counter(ol2) ". ";
+  content: counter(ol2,lower-roman) ". ";
   counter-increment: ol2;
 }
 
@@ -166,7 +166,7 @@
 }
 
 .public/DraftStyleDefault/orderedListItem.public/DraftStyleDefault/depth4:before {
-  content: counter(ol4) ". ";
+  content: counter(ol4,lower-alpha) ". ";
   counter-increment: ol4;
 }