text: Title: Move prompt templates to separate files
  Body: ### 🚀 The feature
  
  Currently we have prompt's content is stored in class body mixed with other code:
  [`GeneratePythonCodePrompt`](https://github.com/gventuri/pandas-ai/blob/a8a15052eff658667d4eeccbe2431bc31911919d/pandasai/prompts/generate_python_code.py#L42)
  [`CorrectErrorPrompt`](https://github.com/gventuri/pandas-ai/blob/a8a15052eff658667d4eeccbe2431bc31911919d/pandasai/prompts/correct_error_prompt.py#L25)
  
  My suggestion is to move those template strings to separate files stored in specific directory. We could implement one more class extending base `Prompt` containing a method that would read content from a file. Then, make `GeneratePythonCodePrompt` and `CorrectErrorPrompt` inherit the new class, and they would only contain an attribute representing a path to the template.
  
  ### Motivation, pitch
  
  This would enhance readibility of sources, segregate business logic from data.
  
  ### Alternatives
  
  _No response_
  
  ### Additional context
  
  _No response_