[terra-form-select] Add exports for subcomponents

[sdadn]

Summary

This PR adds proper exports for subcomponents for convenience and to follow standard programming practices. Before, subcomponents/variants had to be imported by specifying the path to the file. e.g.

import Combobox from 'terra-form-select/lib/Combobox';

With these changes, it can now be imported as:

import { Combobox } from 'terra-form-select';

Note:
This is a passive change and simply adds the ability to use the new imports. Consumes using the old style will not be affected.

Testing

This change was tested using:

• WDIO
• Jest
• Visual testing (please attach a screenshot or recording)
• Other (please describe below)
• No tests are needed

Reviews

In addition to engineering reviews, this PR needs:

• UX review
• Accessibility review
• Functional review

Additional Details

---

Thank you for contributing to Terra.
@cerner/terra