Updating the classnames and adding the @types/classnames npm packages, facilitating TS development

[cassiano]

Fixes #3674

Changes:

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• 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

[raclim]

Thanks for catching this, will merge this in!