Make it possible to have separate instances with separate state and separate task channels

[emassey0135]

This would be good to make sure Minecraft Access and Audio Navigation don't end up changing each other's state, and so they have separate speech queues and one cannot cancel the other, and the same for other instances of multiple mods or applications running in the same JVM, Python instance, etc. This is also important for the C API, to make sure one library cannot take another library's error with whisprs_get_last_error. And this would allow the instance struct to implement Drop, AutoClosable, etc for a more ergonomic interface in some languages.

[emassey0135]

This may be difficult to do with the eSpeak NG backend, because eSpeak NG always has only one instance and its global state is modified frequently, but that can probably be solved with a mutex, or if the API is not thread safe, with mpsc and a background queue thread.

[emassey0135]

This is done except for the C and Java APIs and more testing is needed.