fix: Add generic type support to the bootstrap-4 theme

[nickgros]

Based on #3319, partial fix for #3072

• Adds generic support for Bootstrap 4 theme

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[heath-freenome]

Whoops, you forgot to update the main index.ts

import Form from "./Form/Form";

export { default as Form, generateForm } from "./Form";
export { default as Templates, generateTemplates } from "./Templates";
export { default as Theme, generateTheme } from "./Theme";
export { default as Widgets, generateWidgets } from "./Widgets";

export default Form;