ui: double-click a line ending in a non-terminated ' doesn't highlight the line

Double-clicking \n selects the whole line. Separately, double-clicking after an opening delimiter (like ' or " or whatever) selects until the closing delimiter. If there is no closing delimiter, we currently don't select anything. Instead, if there is no closing delimiter, we should ignore delimiter-based selection and then check whether to select the whole line.

To reproduce:

Create a textbox containing only a single line of text which ends with `'`. Double-clicking after the `'` should select the line. Currently it does not (because it instead looks forward for a closing `'`, finds none, and bails).