#21 When typing string, entering a double quote should close

[Simon Brooke]

When you're typing in the jaberwocky editor window,
typing a double quote character (") causes two double
quote characters to be generated and the caret to be
placed between them (good). You type the string. If
you then type a second double quote character to
terminate the string, the caret should be jumped over
the double quote character that has already been
generated. At present, another two double quote
characters are generated.