feat: add into, optional, const and skip options with enhanced visibility parser (closes #30, #31)

[mahdi739]

This PR introduces into, optional, const, and skip options for related macros.

• into is only allowed for set and set_with. It enables setters to accept std::convert::Into<T> for a field type T.
  Example:

#[derive(Setters, Default)]
pub struct Config {
    #[getset(set = "into")]
    name: String,
}

fn example() {
    Config::default().set_name("val");
}

• optional is only allowed for set and set_with, and the field must be an Option<T> type. It allows setters to accept T instead of Option<T>.
  Example:

#[derive(Setters, Default)]
pub struct Config {
    #[getset(set = "optional")]
    name: Option<String>,
}
fn example() {
    Config::default().set_name("val".to_string());
}

• optional and into can be used together:

#[derive(Setters, Default)]
pub struct Config {
    #[getset(set = "into optional")]
    name: Option<String>,
}
fn example() {
    Config::default().set_name("val");
}

• const makes the related method const. Due to its strict constraints, any compile-time errors that arise when using it must be handled by the user.
  Example:

#[derive(Getters)]
struct MathConstants {
    #[getset(get = "pub const")]
    pi: f64,
}

const PI: &f64 = MathConstants { pi: 3.14159 }.pi();

• skip excludes the attribute individually. It is incompatible with other options and must be used alone for that specific attribute. Note that it's different than the skip attribute.
  Example:

#[derive(CopyGetters, Getters, Default)]
#[getset(get_copy)]
pub struct Config {
    #[getset(get_copy = "skip", get)]
    name: String,
    id: usize,
    number: u32,
}
fn example() {
    let name: &String = Config::default().name();
}

Additionally, the visibility parser has been improved to support more complex visibility specifiers, such as pub(in crate).

[jbaublitz]

Hi @mahdi739, thanks for your contribution. Could you resolve the CI issues? I'll take a look after that.

[mahdi739]

Hi @jbaublitz. I didn't understand the error. But I can resolve the clippy warnings.