This is a follow up to https://github.com/lightpanda-io/browser/pull/2290. That
PR spit ScriptManager into ScriptManager and ScriptManagerBase, with the
goal of letting a Worker have a ScriptManagerBase that works without needing
a frame.
But the Script object was still a large object meant to work with any type of
script invocation, e.g. it has a nullable element. This commit swaps out these
fields for a better-typed "extra" union field. So that Element only exists in
the extra.frame tag. In the end, it just makes it more explicit about what
fields are valid under specific conditions.
a591956dcc