TokenStream should implement Extend

[nox]

impl<T> Extend<T> for TokenStream
where
    T: Into<TokenStream>;

This would allow servo/servo#18340 to be written in a slightly better way.

[Arnavion]

#53304 added Extend<TokenTree> and Extend<TokenStream>

[SimonSapin]

Closing as I think #53304 would be good enough for servo/servo#18340. @nox, feel free to re-open or comment if you think the more generic form would still be useful.