fix: match Transformers `tojson` in chat template rendering (#1486)

* Render chat template tojson filter as unicode

* replace tojson with custom filter

* add indent parameter

* add break, continue and strftime

* renamed to strftime_now

* add separators and sort_keys plus import fix

* allow ensure_ascii parameter as a few templates are using it now

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Co-authored-by: abetlen <abetlen@gmail.com>