- Add `ShadowRoot` to `JSValue` to avoid
`WebDriverJSError::UnknownType`, and
`JavaScriptEvaluationError::SerializationError` when execute JS from
embedder.
- Add unit test.
- Move [is_detached](https://w3c.github.io/webdriver/#dfn-is-detached)
to `fn is_detached` to be reused.
- Other random simplification.
Testing: WebDriver conformance tests.
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Signed-off-by: Euclid Ye <euclid.ye@huawei.com>