add: server chat mode with llama2

[nhamanasu]

Added a script to chat with llama2 in server mode for llama.cpp.

• examples/server-llama2-13B.sh
  • Start the server with llama2 13B.
• examples/server/chat-llama2.sh
  • Script to chat with llama2 when the server is running. Modifications from chat.sh are as follows:
    • Added support for the prompt format of llama2 chat.
    • Fixed an issue where newlines (\n) in llama2 output were not displayed correctly in the terminal.
    • Changed user input to appear in green (like ./main).