script: Support decomposing ShadowRoot from mozjs HandleValue (#38984)

- 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.

---------

Signed-off-by: Euclid Ye <euclid.ye@huawei.com>

components/shared/embedder/lib.rs

@@ -1047,6 +1047,7 @@ pub enum JSValue {
     Number(f64),
     String(String),
     Element(String),
+    ShadowRoot(String),
     Frame(String),
     Window(String),
     Array(Vec<JSValue>),

components/shared/embedder/webdriver.rs

@@ -256,6 +256,7 @@ pub enum WebDriverJSError {
     /// Occurs when handler received an event message for a layout channel that is not
     /// associated with the current script thread
     BrowsingContextNotFound,
+    DetachedShadowRoot,
     JSException(JSValue),
     JSError,
     StaleElementReference,