Add validators and insertables for Oppia
Add validator for variables. Eg: <[assumptions]>,
<[sectionNameI18nId | translate]>
Add insertables for,
- HTML entities: &
- HTML tags: <a href="xyz"></a>
Bug: T237919
Change-Id: I98b97628c5f0a2cd5f6c584a305b66931c32b1cc