Support vi-mode 'InputFormat' prompt

Prompt defining what vi-mode is in, is either 'vi-insert' or 'vi-navigate' (or something 'vi-...' I imagine).
From this `[3:6]` is extracted to get either 'ins' or 'nav' in the prompt.
This is consistent with the behavior I experience when manipulating a custom prompt from the most updated pip installable package.

However, in the case of installing the current master from source, the prompt is on the format 'InputFormat.INSERT' and 'InputFormat.NAVIGATION'.

IPython/terminal/prompts.py

@@ -1,103 +1,108 b''
 """Terminal input and output prompts."""
 
 from pygments.token import Token
 import sys
 
 from IPython.core.displayhook import DisplayHook
 
 from prompt_toolkit.formatted_text import fragment_list_width, PygmentsTokens
 from prompt_toolkit.shortcuts import print_formatted_text
 from prompt_toolkit.enums import EditingMode
 
 
 class Prompts(object):
     def __init__(self, shell):
         self.shell = shell
 
     def vi_mode(self):
         if (getattr(self.shell.pt_app, 'editing_mode', None) == EditingMode.VI
                 and self.shell.prompt_includes_vi_mode):
-            return '['+str(self.shell.pt_app.app.vi_state.input_mode)[3:6]+'] '
+            mode = str(self.shell.pt_app.app.vi_state.input_mode)
+            if mode.startswith('InputMode.'):
+                mode = mode[10:13].lower()
+            elif mode.startswith('vi-'):
+                mode = mode[3:6]
+            return '['+mode+'] '
         return ''
 
 
     def in_prompt_tokens(self):
         return [
             (Token.Prompt, self.vi_mode() ),
             (Token.Prompt, 'In ['),
             (Token.PromptNum, str(self.shell.execution_count)),
             (Token.Prompt, ']: '),
         ]
 
     def _width(self):
         return fragment_list_width(self.in_prompt_tokens())
 
     def continuation_prompt_tokens(self, width=None):
         if width is None:
             width = self._width()
         return [
             (Token.Prompt, (' ' * (width - 5)) + '...: '),
         ]
 
     def rewrite_prompt_tokens(self):
         width = self._width()
         return [
             (Token.Prompt, ('-' * (width - 2)) + '> '),
         ]
 
     def out_prompt_tokens(self):
         return [
             (Token.OutPrompt, 'Out['),
             (Token.OutPromptNum, str(self.shell.execution_count)),
             (Token.OutPrompt, ']: '),
         ]
 
 class ClassicPrompts(Prompts):
     def in_prompt_tokens(self):
         return [
             (Token.Prompt, '>>> '),
         ]
 
     def continuation_prompt_tokens(self, width=None):
         return [
             (Token.Prompt, '... ')
         ]
 
     def rewrite_prompt_tokens(self):
         return []
 
     def out_prompt_tokens(self):
         return []
 
 class RichPromptDisplayHook(DisplayHook):
     """Subclass of base display hook using coloured prompt"""
     def write_output_prompt(self):
         sys.stdout.write(self.shell.separate_out)
         # If we're not displaying a prompt, it effectively ends with a newline,
         # because the output will be left-aligned.
         self.prompt_end_newline = True
 
         if self.do_full_cache:
             tokens = self.shell.prompts.out_prompt_tokens()
             prompt_txt = ''.join(s for t, s in tokens)
             if prompt_txt and not prompt_txt.endswith('\n'):
                 # Ask for a newline before multiline output
                 self.prompt_end_newline = False
 
             if self.shell.pt_app:
                 print_formatted_text(PygmentsTokens(tokens),
                     style=self.shell.pt_app.app.style, end='',
                 )
             else:
                 sys.stdout.write(prompt_txt)
 
     def write_format_data(self, format_dict, md_dict=None) -> None:
         if self.shell.mime_renderers:
 
             for mime, handler in self.shell.mime_renderers.items():
                 if mime in format_dict:
                     handler(format_dict[mime], None)
                     return
                 
         super().write_format_data(format_dict, md_dict)