feature: Make generate function assist generate a function as a constructor if the generated function has the name "new" and is an asscociated function.

[Kohei316]

close #17050
This PR makes generate function assist generate a function as a constructor if the generated function has the name "new" and is an asscociated function.
If the asscociate type is a record struct, it generates the constructor like this.

impl Foo {
    fn new() -> Self {
        Self { field_1: todo!(), field_2: todo!() }
    } 
}

If the asscociate type is a tuple struct, it generates the constructor like this.

impl Foo {
    fn new() -> Self {
        Self(todo!(), todo!())
    } 
}

If the asscociate type is a unit struct, it generates the constructor like this.

impl Foo {
    fn new() -> Self {
        Self
    } 
}

If the asscociate type is another adt, it generates the constructor like this.

impl Foo {
    fn new() -> Self {
        todo!()
    } 
}

[Veykril]

A test case with a new function that takes arguments would be nice as well (which I don't think this change here handles)

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit ff2629d has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit ff2629d with merge 84ef3cf...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 84ef3cf to master...