In `wptexturize()`, allow dashes before and after curly quotes. Example: This is what she said---"Wow that is cool."

Adds unit tests.

Props adamsilverstein, miqrogroove.
Fixes #20342.


git-svn-id: https://develop.svn.wordpress.org/trunk@28726 602fd350-edb4-49c9-b593-d223f7449a82