add get_clip_name, get_animation_name, get_marker_name method in library

[mirsella]

i have also replaced the impl Into<String> by impl AsRef<str> since not all method always need a owned String, and 90% of the time a &str suffice. This allows the users and our methods to not have to clone everything even when not needed, and so its a gain in performance and memory usage.

i've also ran cargo fmt, and allowed the type complexity clippy lint for bevy's systems.

this implement everything discussed in #18, and so it close #18

[merwaaan]

Thank you, that's a great PR :)