Add Html, Css, JavaScript, and Wasm response types

[davidpdrsn]

I think using T: IntoResponse is more flexible than the previous bound. For example you can response with the contents of a file with

Router::new().route(
    "/",
    get(|| async {
        let Ok(file) = tokio::fs::File::open("index.html").await else {
            return Err(StatusCode::INTERNAL_SERVER_ERROR);
        };
        Ok(Html(AsyncReadBody::new(file)))
    }),
);

Also added css, javascript, and wasm, because why not.

We can move Html into axum in 0.7.

Supersedes #1920

[LiamGallagher737]

I just made a crate with a bunch of content types, maybe I could add them here. Most of them are too specific to have here but most of the image ones and files like pdf would be nice to have.
https://github.com/LiamGallagher737/more_axum_responses/blob/main/src/lib.rs

[davidpdrsn]

but most of the image ones and files like pdf would be nice to have

As long they're just wrappers around plain data and don't require other dependencies I'm fine with it. But yeah some of those are quite specialized and probably not worth having.

Can you make a follow up pr when this is merged?