Add a serde feature for (de)serializing Selectors

[jakubslaby09]

Allow Selectors to be (de)serialized with serde.
This will enable devs who rely on serde to use this crate without writing their own wrapper type. One of the use cases is storing selectors in config files when building a program with scraper as a dependency.

Parsing errors are handled as custom serde errors

fn visit_str<...>(self, v: &str) -> Result<Self::Value, E> {
    Ok(Selector::parse(v).map_err(serde::de::Error::custom)?)
}

[adamreichold]

I think the one thing that is worth discussing here is whether we want to commit to this serialization format when the implementation can be done downstream using existing public API. The CSS representation is obviously somewhat canonical, but there might be a more efficient binary representation?

[jakubslaby09]

I feel like the need for a more efficient selector format is rare, so I would provide string serialization as a readable and time-proven default.
In case someone really needs to optimize for speed at the cost of readability and reliability, but still needs to use serde (i.e. when someone stores thousands of selectors, but a different program in a pipeline only supports json), then they can use something like a custom wrapper type with their own implementation.

[adamreichold]

Please have a look at the Clippy lint.