[tldraw] Add spline arrow kind with binding support

## Problem Currently arrows support two kinds: `arc` and `elbow`. Lines support cubic splines through multiple points, but lines don't support bindings. There's no way to create a smooth, curved multi-point arrow that can bind to shapes. ## Proposed solution Add a new arrow kind (`spline`) that works like the line shape's cubic spline mode but with full binding support on both terminals. ### How it should work - A spline arrow would behave similarly to a line shape with `spline: 'cubic'`, producing smooth cubic Bézier curves through control points - Unlike lines, spline arrows would support bindings on their start and end terminals, allowing them to connect to shapes - The arrow would support arrowhead styles on both ends - When bound shapes move, the spline arrow's terminals would update while preserving the curve shape ### Architecture notes Key areas involved: - **Schema**: Add a new `kind` value (e.g., `'spline'`) to `TLArrowShapeProps`, and potentially a `points` property