python: follow-up to Jinja PR

[cebtenzzre]

• Bump the major version because we have made a breaking change to the chat templates
• Require explicit naming of chat_template and system_message parameters (e.g. chat_session(chat_template=...)), to make clear that they are not the same as the old prompt_template/system_prompt
• Implement loading of chat templates from model files when the one from models3.json is not present/available
• Warn if the chat template or system message looks like the old style, using the same regexes as the chat UI
• Fix use of llmodel_model_foreach_special_token before the model is loaded, which was causing an exception when initializing a model
• Mention the possible solution of allow_download=True if the user passed allow_download=False and the model has no built-in chat template
• Provide a model.current_chat_template property to identify which chat template is in use, which may be different from model.config['chat_template'] if overridden or loaded from the GGUF file

I've tested a few different combinations of models which do/don't have a default chat template, do/don't have a chat template in models3.json, and do/don't have an overridden chat template.

I've also tested with some old-style templates with and without warn_legacy=False.