fix: Use struct init shorthand when applicable in fill struct fields assist

[nathanwhit]

This PR tweaks the fill struct fields assist to use the struct init shorthand when a local variable with a matching name and type is in scope.

For example:

struct Foo {
    a: usize,
    b: i32,
    c: char,
}

fn main() {
    let a = 1;
    let b = 2;
    let c = 3;
    let foo = Foo { <|> };
}

Before we would insert

Foo {
    a: (),
    b: (),
    c: (),
}

now we would insert

Foo {
    a,
    b,
    c: ()
}

[matklad]

Lovely 😍, I've been wanting this since forever!

[matklad]

bors d+

[bors]

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[nathanwhit]

bors r=matklad

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)