Improve property validation in web-js

[corbadoman]

Why

To further improve developer experience we must validate properties/options for web-js.

TODOS

• Validate initialBlock in Corbado.mountAuthUI() (for signup-init and login-init for example)
  • If invalid value is given throw an exception with a message like "Invalid initialBlock 'xxx' given (allowed values: 'a', 'b')"