[feat] Convenient access (using macro)

TODO:
- [ ] Define a macro (`@XConfigs` or `@XConfigSpec` replacing the protocol.)
- [ ] Automatically create the default instance
- [ ] Automatically create static variable to access the default instance.

Example:

From:

```swift
AppConfigs.shared.flagA
```

To automatically produce this:

```swift
AppConfigs.flagA
```