Add missing 'PlayDialog-turbo' to VoiceEngine options

[FAHADPN]

Description:
The 'PlayDialog-turbo' voice engine was missing from the VoiceEngine type definition, even though the model is supported by the package. This update adds the missing option to ensure accurate type coverage and prevent potential runtime mismatches.

Changes Made:

export type VoiceEngine = 'PlayDialog' | 'PlayDialog-turbo' | 'Play3.0-mini' | 'PlayHT2.0-turbo' | 'PlayHT2.0' | 'PlayHT1.0' | 'Standard';

[FAHADPN]

solves issue #64

[acdcjunior]

Hey, thanks for this PR. This is a good change, although I'm curious what was your use case, where this VoiceEngine type made a difference?

I ask because, per this test, the PlayDialog-turbo engine is available on the PlayHT.stream method

[FAHADPN]

Thanks! The main reason for this change was to make the mapping between my custom model and voice engine explicit and type-safe. While PlayDialog-turbo works with PlayHT.stream, it was missing from the VoiceEngine type, which caused issues like type errors or missed autocomplete. I was building a helper function to map my custom models to engines, and having a complete and accurate VoiceEngine type helped ensure consistency and avoid potential mismatches at runtime.