feat: add typescript declarations

[aarongranick-okta]

• no changes for users of the module who are not using typescript
• if using typescript, esModuleInterop is required in your tsconfig.json, since this a commonjs module which exports a function (class constructor).
• types in the library are available in the OktaAuth namespace, which is merged with the OktaAuth class declaration. For example:

import OktaAuth from '@okta/okta-auth-js'
const config:OktaAuth.OktaAuthConfig = {...};
const oktaAuth:OktaAuth = new OktaAuth(config);
const tokenManager:OktaAuth.TokenManagerAPI = oktaAuth.tokenManager;

[ekasprzyk]

Just a reminder that this would be nice to have done, please.

[aarongranick-okta]

typescript was shipped with [email protected]