Limit input attributes to inputs with a valid `type` attribute

[anp]

Example: only file inputs should statically allow the accept attribute, i.e. this should be a compile error:

input().type_("checkbox").accept("*").build()

I think this can be done by making Input<Ty: InputType> and some zero-sized types which implement InputType. Then you'd limit the impls to only the input types that are valid:

impl Input<File> {
    fn accept(self, a: &str) -> Self { ... }
}

To do this, we need the type_() function on all elements to "fill in" the generic parameter. The question I have right now is how to get the right type info at compile time while preserving the same mox syntax as we currently have:

mox! { <input type="file" accept="*"></input> }

// expands to
input()
    .type_("file")
    .accept("*")
    .build()

One possible approach would be to use a const generic string once thats available:

input()
    .type_::<"file">()
    .accept("*")
    .build()

Another would be to make the function generic over a ZST argument, but lose the "stringness" of the type attribute:

input()
    .type_(File)
    .accept("*")
    .build()

Both of these APIs could be supported with the same surface syntax in mox but adding a bunch of web-specific constants isn't great when the macro should work across platforms.

If we don't special-case the macro and we require a different syntax for invoking the type, then we lose a bit of HTML/XML familiarity which is some of the point of the macro in the first place.