Feature request: Add field to struct / enum variant definition

Example input for struct:

struct Foo {
    bar: u8,
}

let foo = Foo {
    bar: 0,
    baz$0: "test",
};

After 'Add field to struct definition':

struct Foo<'a> {
    bar: u8,
    baz: &'a str,
}

let foo = Foo {
    bar: 0,
    baz: "test",
};

Example input for enum:

enum Foo {
    Bar,
    Baz,
}

let foo = Foo::Bar(5$0);

After 'Add field to enum variant':

enum Foo {
    Bar(i32),
    Baz,
}

let foo = Foo::Bar(5);

Start with the struct and enum examples in the issue, then trace rust-analyzer's existing assist behavior for related definition changes. Done means both requested transformations work from the marked expression and produce the shown struct-field and enum-variant definitions while updating the expression accordingly.