This is to make pasting literals easier.
When a user pastes something, we normally take it as-is.
The exception is when a single-quote is open, e.g. the current token
is
foo'bar
When something is pasted here, we escape single-quotes (`'`) and
backslashes (`\\`), so typing a `'` after it will turn it into a
literal token.
Fixes#967.
