Export CursorContext (#259)

## Summary:

@khanacademy/math-input clients sometimes use the CursorContext as part of using the math-input project. Instead of re-defining the values in the object, this PR adds it as an export from the main module.

Issue: MOB-4525

## Test plan:

Author: jeremywiebe

Reviewers: kevinbarabash

Required Reviewers:

Approved By: kevinbarabash

Checks: ✅ Lint, Flow, and Test (ubuntu-latest, 16.x), ✅ Extract i18n strings (ubuntu-latest, 16.x), ✅ Check builds for changes in size (ubuntu-latest, 16.x), ✅ Check for .changeset file (ubuntu-latest, 16.x), ✅ Cypress Coverage (ubuntu-latest, 16.x), ✅ Jest Coverage (ubuntu-latest, 16.x), ❌ codecov/project, ✅ gerald, ✅ gerald

Pull Request URL: #259

.changeset/happy-windows-repair.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/math-input": patch
+---
+
+Export CursorContexts

packages/math-input/src/index.js

@@ -12,3 +12,7 @@ export {
 } from "./components/prop-types.js";
 export {default as Keypad} from "./components/provided-keypad.js";
 export {KeypadTypes} from "./consts.js";
+
+import * as CursorContexts from "./components/input/cursor-contexts.js";
+
+export {CursorContexts};