New assist: Generate `From` implementations from `from()` or `into()` calls.

Given a B::from(a) or a.into() call which is currently erroring with an unsatisfied From<A> or Into<B> bound, this assist would generate the skeleton of the appropriate impl:

impl From<A> for B {
    fn from(value: A) -> Self {
        todo!()
    }
}

A further elaboration might be to sketch the body:

• If A is an enum, automatically insert a match value { <arms> }.
• If A is a struct with public fields, automatically insert let A { <fields> } = value;.
• If B is a struct with public fields, automatically insert Self { <fields> }.

This assist would greatly speed up the process of writing type conversion boilerplate, and would make writing Froms at least as easy as writing plain functions with the “Generate function” assist.

Start by examining the existing “Generate function” assist referenced in the issue and how rust-analyzer assists are structured. Implement generation of the From<A> for B skeleton for failing from() or into() calls, then consider whether the enum and struct body elaborations belong in the same scope; done means the assist produces the requested implementation.