refactor: export CanvasContextProvider to follow established context pattern

As noted [in the review](
https://github.com/drawdb-io/drawdb/pull/148#discussion_r1681584836
), I disagree with this pattern. However, I will follow it to get
these features added.

src/hooks/index.js

@@ -1,4 +1,5 @@
 export { default as useAreas } from "./useAreas";
+export { default as useCanvas } from "./useCanvas";
 export { default as useLayout } from "./useLayout";
 export { default as useNotes } from "./useNotes";
 export { default as useSaveState } from "./useSaveState";

src/hooks/useCanvas.js

@@ -0,0 +1,6 @@
+import { useContext } from "react";
+import { CanvasContext } from "../context/CanvasContext";
+
+export default function useCanvas() {
+  return useContext(CanvasContext);
+}