Better Rail Curve logic -- player chosen, no south-east rule

Right now, when a rail is adjacent to three other rails, it is always curved, and always curve towards south and east, unless powered when they curve away from south and east.

This idea is to make rail curvature selected by the builder:

If the player was not holding sneak when they placed the rail, it's natural curve direction is clockwise.

If the player was holding sneak when they placed the rail, it's natural curve direction is counterclockwise.

If a curved rail becomes redstone powered, it will curve in the opposite direction from it's "natural" direction is, going from clockwise to counterclockwise or counterclockwise to clockwise.

As a result of this change, builders can create static builds with curved rails without needing redstone, but when dynamic curve selection is needed, redstone can still do the job.

If a three-way intersection is created by non-player means, such as pistons, the natural curve direction should be clockwise, instead of following the south-east rule.

If this is implemented, rail blocks can being upgraded infer their natural curve direction from how they are currently curved and whether they are currently redstone powered.  Most rail builds should not break, except perhaps if they depend on pistons moving rails around to create curved rails at three way intersections.

As a quality of life feature, if a player in creative mode right clicks a curved rail with an empty hand, it's natural curve direction should be reversed.