Only set radius if the collision shape has that property, to allow things like boxes or polygons for SnapPath.

Adds to 'picatinny_scope' and sets up 'picatinny_rail' to be more accurate.

Previously adding a snap_interval would move objects one snap_interval forward and prevent objects from snapping backwards over the edge of the path. This solution checks and solves for this edge case. If the object were to snap its front (p1) further back than the max-backwards position (path_p2 + _length) it will round up to the nearest snap_interval, moving it forwards.
- Add clarifying comments

more_snap should have been given p1_new instead of ideal_snap. Previously both values would be the same. Now works as expected, the PicatinnyScope in the demo scene will snap forward on the rail instead of overhanging when placed at the very back of the rail.

When a pickable object is placed on a snap_path the pickable's CollisionShape3Ds are used to detect grabbing from the rail. This behavior is more intuitive for the class user and prevents desync between a temp snap_zone's automatic circle shape and the pickable. It will be more computationally expensive depending on the pickable's shapes.

- Remove errors from missing expected children by using has_node() before get_node()

- Shorten descriptions
- Reorder 'extends'
- Remove whitespace
- Remove unused function
- Remove unnecessary else: return
- Fix 'start_xr.gd' static variable declared out of order
     - Unsure if this fix should be in this PR, but it clears the last of the gdlint errors even though this file was not previously changed in this PR.