[TS] Use strict mode in LGraph (#579)

* [TS] Undefined is not a valid object key

* Deprecate unused code

Adds ts-expect-error to TS strict issues on unmaintained public interfaces.

* [TS] Fix nullability: asSerialisable return type

* nit - Remove outdated comment

* [TS] Strict mode: LGraph

- Adds minor coercion changes; would need extremely specific hacks performed to result in runtime change

* [TMP] Work around typescript-strict-plugin

Must be reverted once plugin removed.
See #578

src/LGraphCanvas.ts

@@ -465,7 +465,7 @@ export class LGraphCanvas implements ConnectionColorContext {
   node_in_panel?: LGraphNode
   last_mouse: ReadOnlyPoint = [0, 0]
   last_mouseclick: number = 0
-  graph!: LGraph
+  graph!: LGraph | null
   canvas: HTMLCanvasElement
   bgcanvas: HTMLCanvasElement
   ctx?: CanvasRenderingContext2D