Use proc macro to define Config

[topecongiro]

This PR implements a proc macro for defining Config struct, which replaces the current define_config macro.

The ultimate goal is to define the config like the following:

#[rustfmt_config]
pub struct Config {
    /// A maximum width of each line.
    ///
    /// - Default value: 100
    /// - Possible values: any positive integers
    /// - Stable: Yes
    ///
    /// See also err_on_line_overflow.
    ///
    /// ## Example
    ///
    /// ### Input
    ///
    /// ```rust
    /// // An example code.
    /// ```
    ///
    /// ### 100 (default)
    /// ```rust
    /// // An example code formatted with max_width = 100.
    /// ```
    ///
    /// ### 80 (default)
    /// ```rust
    /// // An example code formatted with max_width = 80.
    /// ```
    #[rustfmt::config(default = 100, stable = "1.0.0")]
    max_width: usize,

    // ... other configs
}

and let the config proc macro generate Configurations.md and test code under test/source/configs/ and tests/target/configs/. This PR is the first step toward the goal.

Other minor benefits include the following:

• a better interface for setters
• internal config options are now private

Since the Config is part of the public API of rustfmt-as-a-lib, and this PR changes its interface, we should merge this as a part of the preparation for 2.0 release. I am creating a PR at the moment, however, for early feedback.

[calebcartwright]

This will resolve #3872 correct?

[topecongiro]

Unfortunately no, IIRC the issue is that we are parsing the same rustfmt.toml and emitting errors multiple times.

[calebcartwright]

👍 definitely a friendly interface on getters/setters

[topecongiro]

I will try this again after 2.0 release 🤷‍♂️