Tweak auto-parens behavior of double quote typed in a string
so that it results in the string split into two strings or,
in case part of the string is already selected, three strings,
where the selection is retained in the latter situation.
Improve auto-parens mode implementation and behavior
including behavior of double quote typed in a string
so that it results in the string split into two strings or,
in case part of the string is already selected, three strings,
where the selection is retained in the latter situation.
Also redid implementation of insertion of open+close characters
so that it doesn't require adding in tokens to the editor,
testing the resulting state, and then undoing that to decide
whether to insert the pair or just the opening symbol.