whisper : make voice_length() utf-8 aware for CJK (#3915)

whisper : make voice_length() utf-8 aware for CJK (#3915)

* whisper : make voice_length() utf-8 aware for CJK

voice_length() weights each token by how long its text takes to say, which drives
how a segment's time is shared between its tokens. It looped over raw bytes, so
every CJK character (3 bytes) was counted ~3x and full-width punctuation never
matched, skewing token timestamps for Chinese/Japanese.

Decode one utf-8 code point at a time and give full-width ，。！？ etc. the same
weights as their ASCII counterparts. Pure-ASCII text is unaffected.

* whisper : one statement per line in voice_length()

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Co-authored-by: linxiaodong <redacted>