Add typecheck into pull request template

Author: clairep94

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,6 +6,7 @@ I have verified that this pull request:
 
 * [ ] has no linting errors (`npm run lint`)
 * [ ] has no test errors (`npm run test`)
+* [ ] has no typecheck errors (`npm run typecheck`)
 * [ ] is from a uniquely-named feature branch and is up to date with the  `develop` branch.
 * [ ] is descriptively named and links to an issue number, i.e. `Fixes #123`
 * [ ] meets the standards outlined in the [accessibility guidelines](https://github.com/processing/p5.js-web-editor/blob/develop/contributor_docs/accessibility.md)