1. removed ScriptThread::get_safe_cx in favor of Runtime::cx (this intentionally shadows RustRuntime::cx, which is not really safe with all other stuff that happen in servo).
2. passed down cx from ScriptThread::new to accommodate those changes (we also need to pass cx for some other stuff).
3. we create JSContext in workers/worklets, so we can pass all the way down to

   servo/components/script/dom/workers/workerglobalscope.rs

   Line 1013 in 6ccc47f

   CommonScriptMsg::Task(_, task, _, _) => task.run_box(),

   (so we can get &mut JSContext in tasks).

So with this PR, we prepared all entry points of JSContext (or other should actually be reentering via SM callback or passed down from the root one in the thread).

Testing: No changes just safety, but it should be covered by WPT
Part of #40600