Remove LayoutElement, resolve root TS issues (#953)

- Converts type assertions to use inference via discriminated unions
- Removes the LayoutElement class (only used by node slots, and recently
reduced to a single function)
- Splits `boundingRect` property out from `Positionable` interface
- Slots now use the standard `boundingRect` property
- Perf improvements / Removes redundant code

src/utils/layout.ts

@@ -1,18 +0,0 @@
-import { Point, ReadOnlyRect } from "@/interfaces"
-
-export class LayoutElement {
-  public readonly boundingRect: ReadOnlyRect
-
-  constructor(o: {
-    boundingRect: ReadOnlyRect
-  }) {
-    this.boundingRect = o.boundingRect
-  }
-
-  get center(): Point {
-    return [
-      this.boundingRect[0] + this.boundingRect[2] / 2,
-      this.boundingRect[1] + this.boundingRect[3] / 2,
-    ]
-  }
-}