Add configuration for vim

- Add ~/.config/vim/vimrc
  + Configure basic stuff
  + Activate spell checks
  + Activate syntax highlighting and ruler
  + Show whitespace characters
  + Add various snippets for mouse handling,
    toggling line numbers, and search
  + Set `vim`-related directories inside
   ~/.config/vim and ~/.local/state/vim
- Integrate extra settings for Python files
  in ~/.config/vim/after/ftplugin/python.vim
  and ~/.config/vim/after/syntax/python.vim
- Add `vim` artifacts in ~/.config/git/ignore

.config/shell/welcome

@@ -5,6 +5,7 @@
 
 # ... except:
 [ -t 1 ] || return                  # If stdout is not a terminal
+[ "${SHLVL:-1}" -gt 1 ] && return   # In nested shells (e.g., ":terminal" in `vi`)
 [ "${TERM:-}" = "dumb" ] && return  # For tools opening shells (e.g., `emacs`)