Don't include trailing whitespace in preserved selection

[munificent]

Right now, if the end of the selection is in whitespace, it will be pushed to the beginning of the next non-whitespace token when the text is formatted.

I think a user will intuitively expect the selection to rubber-band around the selected text which means moving the selection start forward past whitespace and the selection end back before it. (We also have to handle the case where both the selection start and end are in the middle of the same whitespace.)