Add options for cardinal/ordinal plural types

[oliver-ni]

Adds functionality for switching between plural types (cardinal and ordinal).

Fluent docs include this option in the NUMBER built-in function. Since fluent-rs doesn't have that yet, I included it in the place where all the other number options were. Please let me know if this isn't the appropriate place / way to include this functionality.

Also, I called it kind instead of type since the latter is a Rust keyword... a little weird but not sure if there's a better way to deal with that.

[juliancoffee]

@oliver-ni to answer on using "type" in Rust, you can use raw identifiers. https://doc.rust-lang.org/rust-by-example/compatibility/raw_identifiers.html

[gregtatum]

Closing as stale. Feel free to re-open if you want to address the requested changes.