Tilde \textasciitilde and fix ~ in \verb (#1286)

* \textasciitilde and fix ~ in \verb

* Add support for `\textasciitilde` which reproduces tilde character
  in text mode.
* Make `~` render as tilde instead of space within `\verb`,
  by manually mapping that character to `\textasciitilde`.
  This seems to be the only character that needs such manual mapping
  to reproduce LaTeX's `\verb` behavior.

* Add `, -, ', ~ to Verb screenshot test

test/screenshotter/ss_data.yaml

@@ -358,6 +358,7 @@ Verb: |
       \verb* \verb* , & \verb*|\verb* |, \\
       \verb!<x> & </y>! & \scriptstyle\verb|ss verb| \\
       \verb*!<x> & </y>! & \small\verb|sm verb| \\
+      \verb|``---''~|
     \end{array}
 VerticalSpacing:
     pre: potato<br>blah