v0.28.0

Major refactor to improve ease of maintenance

• Move the session to an async task, using channels.
• Remove WebDriverCommands trait, and add methods directly to SessionHandle. This also makes docs far more readable.
• Store SessionHandle in WebDriver directly and remove GenericWebDriver. WebDriver is no longer generic.
• Pass a reference to SessionHandle to all elements and other structs that make use of the session. This prevents use of elements after the browser is closed (at compile time).
• Simplify ElementQuery slightly (removed single selector option).
• Move all session handling to the session module.
• Update dependency versions.