Improve LTN Turn Restriction editing UX design

Based on feedback here: https://github.com/a-b-street/abstreet/pull/1091#pullrequestreview-1604775328

> The UX of only being able to select a destination road on
one end is confusing to me. When you hover on a road in the neutral
state, roads on both endpoints are highlighted, but when you click, the
closer one wins.

## Option 1

> My first preference might be to get rid of the endpoint distinction
and just let the destination road exist on either end. I remember
discussing this and don't remember the rationale... was it due to
ambiguity with complex TRs and sequences of multiple roads where some
road might be shared?

This would require finding a solution for https://github.com/a-b-street/abstreet/issues/1108

## Option 2

> As a second option, what if we change the hover behavior to make it
clear which side of the road the cursor is closer to, so the click
becomes expected?

This would require calculating the hitboxes for each half-road segment. This would be a faff, but it is possible.