[LEMS-2563] Primary widget splitter (#2234)

## Summary:
Create a central place to pass in a PerseusItem and return a version with all the widget options split of answer data.

Mostly tests and some moving of logic. The main important thing is `splitPerseusItem`.

Issue: LEMS-2563

## Test plan:
This just adds the logic, still not using it. Just unit tests right now.

Author: handeyeco

Reviewers: handeyeco, benchristel, jeremywiebe, Myranae

Required Reviewers:

Approved By: benchristel

Checks: ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x)

Pull Request URL: #2234

.changeset/sixty-balloons-brake.md

@@ -0,0 +1,6 @@
+---
+"@khanacademy/perseus": major
+"@khanacademy/perseus-core": minor
+---
+
+Move splitters into perseus-core, add splitPerseusItem

packages/perseus-core/src/index.ts

@@ -119,6 +119,8 @@ export {
     upgradeWidgetInfoToLatestVersion,
 } from "./widgets/upgrade";
 
+export {default as splitPerseusItem} from "./utils/split-perseus-item";
+
 export type * from "./widgets/logic-export.types";
 
 export * as CoreWidgetRegistry from "./widgets/core-widget-registry";