Add get_encoding_name_for_model to tiktoken

[noseworthy]

The tiktoken-js library includes a very helpful function,
getEncodingNameForModel(). This function is buried in the
implementation of encoding_for_model() in the rust based
tiktoken package.

This function is very useful when implementing an encoding cache based
on the model used. In this case, having a mapping from model ->
encoding and then caching based on the encoding name conserves
resources since so many models re-use the same encoding.

I've exposed a new get_encoding_name_for_model() function
that behaves similarly to the one in the tiktoken-js package, and used
it inside of encoding_for_model().

Finally, I've also added a test to ensure that this function can be
called properly from typescript code, and that it properly throws
exceptions in the case of invalid model names.

Fixes: #123

[noseworthy]

Hey, @dqbd and @jens-f 👋

I think this should fix #123. It's a feature we've been looking for as well, so I figured I'd take a crack at it.

I'm not a rust developer, so please excuse any obvious blunders on my part. I'd love to know what you think. It'd be awesome to have this functionality exposed!

Thanks in advance for your consideration of the PR 🙏