feat: add assist: extract methods to super trait

Example

trait Foo {
    fn foo();
    $0fn bar();
    fn baz();$0
}
impl Foo for () {
    fn foo() { println!("a") }
    fn bar() { println!("b") }
    fn baz() { println!("c") }
}

->

trait $0SuperTrait {
    fn bar();
    fn baz();
}
trait Foo: SuperTrait {
    fn foo();
}
impl SuperTrait for () {
    fn bar() { println!("b") }
    fn baz() { println!("c") }
}
impl Foo for () {
    fn foo() { println!("a") }
}

No implementation file or test is named. Start by locating rust-analyzer's existing assist implementations and tests, then compare the requested example transformation with the current assist structure. Done means the assist extracts the selected methods into a new super trait and updates the original trait and impls as shown.