New assist: change multiple `let` assignments of same variable to a `let` block

I'd like to propose an assist that converts a common pattter of variable initialization by multiple let assignments of same variable (not sure if the pattern has a name) into a let x = {...}; block. I think it often assists with readability, especially if the reassignments are more complex.

This:

let is_even = "123";
let is_even: u32 = is_even.parse()?;
let is_even = is_even % 2 == 0;

into this:

let is_even = {
    let is_even = "123";
    let is_even: u32 = is_even.parse()?;
    is_even % 2 == 0
};

and back.

Start by reviewing rust-analyzer's existing assist framework and how assists handle transformations in both directions. Use the examples in the issue as the target behavior: combine repeated let assignments into a let block and convert that block back when applicable. Done means both transformations preserve the shown bindings and expressions.