Add previous voice toggle functionality

[GruiaChiscop]

Introduces a script and supporting method to switch to the previous voice in the voice settings. Updates gesture descriptions and ensures proper index handling for both next and previous voice toggling. The voice toggle has also been added in the NVDA's input gestures dialog

[adamsamec]

At this point, I don't plan to merge this PR mainly because of two reasons:

1. You are introducing logic to the init.py file which should be instead placed into voiceToggle.py.
2. You have removed the default gesture - NVDA + Alt + V. VoiceToggle should not expect user to need to define their own gesture.