Add shorthand field completion for record-expr

[A4-Tacks]

Close #20830

Example

struct Foo { bar: bool, n: i32 }

fn baz() {
    let bar = true;
    let foo: Foo = Fo$0;
}

Before this PR:

struct Foo { bar: bool, n: i32 }

fn baz() {
    let bar = true;
    let foo: Foo = Foo { bar: ${1:()}, n: ${2:()} }$0;
}

After this PR:

struct Foo { bar: bool, n: i32 }

fn baz() {
    let bar = true;
    let foo: Foo = Foo { bar$1, n: ${2:()} }$0;
}