[tldraw] Add cast-shadow property to shapes

## Problem statement There's currently no way to add a visual shadow or elevation effect to shapes. Shadows are a common design element for creating depth, visual hierarchy, and emphasis in canvas-based designs. Currently, the only shadow-related code is an internal utility (`rotated-box-shadow.ts`) used specifically for embed/bookmark shapes — it's not exposed as a general-purpose shape property. ## Proposed solution Add a new style property (or core shape property like opacity) that applies a CSS `box-shadow` or SVG filter-based shadow to any shape, giving it a cast-shadow / elevation appearance. ### Approach options **Option A: Enum-based `StyleProp` (like fill/dash)** Define a set of discrete elevation levels: | Value | Description | |-------|-------------| | `none` | No shadow (default) | | `sm` | Subtle shadow — slight lift | | `md` | Medium shadow — card-like elevation | | `lg` | Prominent shadow — floating/modal feel | This is consistent with how other visual properties