Add partial selection for generate_getter_or_setter

[A4-Tacks]

Adding a partial of the selected field can also generate getter for it

This makes the selection more convenient

Example

struct Context {
    data$0: Data,
    count$0: usize,
    other: usize,
}

Before this PR

Assist not applicable

After this PR

struct Context {
    data: Data,
    count: usize,
    other: usize,
}

impl Context {
    fn data(&self) -> &Data {
        &self.data
    }

    fn $0count(&self) -> &usize {
        &self.count
    }
}