feat: input-box prompt

[swit33]

This PR introduces an interactive "ask" functionality to PopAI. Currently, the plugin relies on pre-defined static prompts. This change allows users to provide real-time instructions/questions via vim.ui.input(), which are then injected into the LLM request.

Key Changes

• Interactive Input: Added logic to detect an ask action which triggers a native Neovim input box.
• Dynamic Placeholders: Introduced the {user_prompt} placeholder.
  • If {user_prompt} is present in the template, the user's input is injected at that specific location.
  • If absent, the user's input is appended to the end of the template for a natural "Instruction -> Context" flow.
• Fallback Logic: Maintains backward compatibility with existing static templates.