Add a suffix string to the class selector of the generated CSS rule.
### Example
```
use encre_css::{Config, generate};
use encre_css::prelude::build_plugin::*;
pub(crate) const PLUGIN: StaticPlugin = Plugin::Spacing(Spacing {
namespace: "custom-divide",
prop: SingleProp("margin-inline"),
extra_class: Some(" > :not(:last-child)"),
..Spacing::default()
});
let mut config = Config::default();
config.register_plugin(&PLUGIN);
let generated = generate(["custom-divide-2"], &config);
assert!(generated.ends_with(".custom-divide-2 > :not(:last-child) {
margin-inline: 0.5rem;
}"));
```
### Example in TOML
```toml
[[custom_plugins]]
[custom_plugins.Spacing]
namespace = "custom-divide"
prop = "margin-inline"
extra_class = " > :not(:last-child)"
```