tokenizer : special token handling (#3538)

* Rewrite special token handling from #1931

* shorten param name, add st verification by type

* use offsets instead of copy by substr

* formatting, remove copying iterator on delete

* llama : normalize code-style

* swift fix

* print pfx/sfx if verb, main: split pfx input sfx

* dont add space when using special tokens

* minor : comment + spacing

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>