Inline JS-to-JS frames no longer live in the raw execution context
vector, so LibWeb callers that need to inspect or pop contexts now go
through VM helpers instead of peeking into that storage directly.
This keeps the execution context bookkeeping encapsulated while
preserving existing microtask and realm-entry checks.
Split JS::ErrorData out of JS::Error so that it can be used both
by JS::Error and WebIDL::DOMException. This adds support for
Error.isError to DOMException, also letting us report DOMException
stack information to the console.
