Migrate to new tekken crate

[jorge-menjivar]

I am the author of tekken-rs and recently renamed it to simply tekken to make it easier to import, find, and align with its library name. Since crates.io has no rename mechanism, this meant publishing a new crate; tekken-rs 0.1.2 is the final release under the old name.

Version 0.2.0 of tekken also adds:

• Support for v15 of the tokenizer
• Image tokenization support
• Audio and image dependencies gated behind feature flags

Two things to note:

• No code changes are needed: tekken-rs's library name was already tekken, so all existing use tekken::... imports will keep working
• The existing default-features = false was a no-op on 0.1.1 (it had no feature flags, so the audio dependencies were unconditional), but on 0.2.0 it takes effect: since this project only uses tekken for text tokenization, hound, rubato, rustfft, and ndarray drop out of the dependency tree, and the new image feature is never pulled in.