Description of the new feature/enhancement

For the intuitive highlighting behavior, and consistency with most other applications; when selecting text with the mouse, the two endpoints should be rounded to the nearest vertical boundary between adjacent characters, and highlight/select whatever is between these two points; rather than necessarily including the character the mouse was clicked on / released at.

Note that this is for single click only. For double click, it's the characters under the click location and the release endpoint that should be expanded.

For more details, rationale etc. see for example kovidgoyal/kitty#945.