Skip unknown phonemes in g2p processing

[madest92]

Fix KeyError when unknown phonemes are encountered in g2p methods

# vosk-tts -n vosk-model-tts-ru-0.7-multi -s 3 -o out_vost.wav -i "Found special symbol @"
Loading model from /root/.cache/vosk/vosk-model-tts-ru-0.7-multi/model.onnx

Traceback (most recent call last):
.....
  File "/data/.venv/lib/python3.11/site-packages/vosk_tts/synth.py", line 235, in g2p_noembed
    phoneme_ids.extend(phoneme_id_map[phonemes[i]])
                       ~~~~~~~~~~~~~~^^^^^^^^^^^^^
KeyError: '@'

The problem is observed in all models.

[madest92]

@nshmyrev hi.

Can you take a look?