Add compile error when using comment and doctype tags within the `view!` macro

[thinety]

Discussed in https://github.com/leptos-rs/leptos/issues/2338.

Preview of the error messages:

The first commit contains just style changes (using proc_macro_error::abort! everywhere instead of just abort!, to be consistent with proc_macro_error::emit_error!), so it can be reverted if desired.