Improvement: Add type annotations to improve code readability

[Tahaa-Dev]

Most of the ResExt proc-macro (resext-macro crate in resext-macro/src/lib.rs) has no type annotations, which can make code ambiguous and reduce readability, as this snippet:

let alias = args.alias.unwrap_or_else(|| {
    String::from("Res")
});
let alias = quote::format_ident!("{}", alias);

can be ambiguous, while this snippet:

let alias: String = args.alias.unwrap_or_else(|| {
    String::from("Res")
});
let alias: proc_macro2::Ident = quote::format_ident!("{}", alias);

is much more readable especially to new contributors