Implement `SafeFromJSValConvertible`, a safe wrapper for
`ToJSValConvertible`. And, replace unsafe `ToJSValConvertible` with
`SafeFromJSValConvertible` in `script/dom` to reduce the amount of
unsafe code in `script`.
This would support the implementation of `AdoptedStylesheet` where we
will need to have a setter/getter of sequence, that was implemented by
`any` types.
Part of https://github.com/servo/servo/issues/37951
Signed-off-by: Jo Steven Novaryo <jo.steven.novaryo@huawei.com>