This PR aims to improve the abstraction around sending keystrokes/values to input fields. The API aims to match playwright's implementation to allow a more straightforward migration in the near future. There are a few pieces:

1. Add the 'Key' constants to the driver itself, hardcoded instead of using selenium's. These can be later replaced with 'Enter' and 'Backspace' to match the playwright API for .press
2. Add driver.fill API to the driver that matches playwright's fill method. Fill differs from sendKeys in that it replaces the current value instead of adding to it.
3. Add driver.press API to the driver that matches playwright's press method. Playwright's implementation differs in that it expects key names like 'Enter' while selenium's expects unicode encoded key values.
4. Wraps all elements returned from driver with fill and press methods.

This PR includes the refactor to use these.