Add multi language support for edit subcommand

[YousefHadder]

Problem

The current config supports one language for edit subcommand, and per #187, using edit -l resets some options in the config file.

Suggested state

I would love to see support for multiple languages in the config file as follow:

[code]
  editor = 'nvim'
  editor_args = []
  editor_envs = []
  default_lang = 'rust'  # default language when no -l flag
  edit_code_marker = false
  test = true

  # Multi-language configurations
  [code.languages.rust]
  lang = 'rust'
  comment_leading = '//'
  inject_before = []
  inject_after = []

  [code.languages.python]
  lang = 'python3'
  comment_leading = '#'
  inject_before = ['#!/usr/bin/env python3']
  inject_after = []

  [code.languages.cpp]
  lang = 'cpp'
  comment_leading = '//'
  inject_before = ['#include <iostream>', '#include <vector>']
  inject_after = []