feat: support user defined macro snippet for completion

[A4-Tacks]

This feature can improve the completion details of macros

And this feature can be utilized to implement excellent DSL completion

Example

#[rust_analyzer::macro_style(snippet = "${path}!($1, $2)")]
macro_rules! assert_eq { ($($t:tt)*) => {} }

fn main() { a$0 }

Before this PR

fn main() { assert_eq!() }

After this PR

fn main() { assert_eq!($1, $2) }

---

macro_rules! html {
    (<div>$($t:tt)*) => {
        /* ... */
    };
    ($($t:tt)*) => {{
        #[rust_analyzer::macro_style(snippet = "<div>\n    $1\n</div>")]
        macro_rules! div { () => {} }
        $($t)*
    }}
}

fn main() {
    html! {
        d$0
    }
}

->

fn main() {
    html! {
        <div>
            $1
        </div>
    }
}

[ChayimFriedman2]

This feature needs design discussions.

Theoretically, if we make it clear that there are absolutely no stability guarantees, it wouldn't, but I suspect people will still complain, so I don't want that.

[A4-Tacks]

This feature needs design discussions.

On zulip? or this PR?

[ChayimFriedman2]

Zulip is better.

[A4-Tacks]

Ok: https://rust-lang.zulipchat.com/#narrow/channel/185405-t-compiler.2Frust-analyzer/topic/User.20defined.20custom.20snippet.20of.20macro.20design/with/614965108