Integrate Popular Macro Assists.

[wyatt-herkamp]

Related Issue #13341

I am proposing we build in some assists and other intellisense features for popular macros.

Proposed Macros/ Libraries we intergrate.

• thiserror
• serde
• Clap
• Possibly sea_orm, sqlx, and diesel

What I would like to see is some assist on Attribute inputs. For example with serde

#[derive(Serialize)]
struct {
	#[serde(<|>)] // Suggestions like default, with, ....
	my_field: u8
}

All the expansion would still happen in the compiled proc_macro but this would allow for hinting.

Another example would be with thiserror
As mentioned in #13341. If the return type doesn't implement From<SomeError> we could suggest adding the value to the return type if it was an enum.

This will not be a perfect solution to the problem of proc macro intellisense, But it would make using some of these very popular libraries a little easier.