Add WN entry for vi prompt stripping

docs/source/whatsnew/pr/vi-prompt-strip.rst

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+Automatic Vi prompt stripping
+=============================
+
+When pasting code into IPython, it will strip the leading prompt characters if
+there are any. For example, you can paste the following code into the console -
+it will still work, even though each line is prefixed with prompts (`In`,
+`Out`)::
+
+    In [1]: 2 * 2 == 4
+    Out[1]: True
+
+    In [2]: print("This still works as pasted")
+
+
+Previously, this was not the case for the Vi-mode prompts::
+
+    In [1]: [ins] In [13]: 2 * 2 == 4
+       ...: Out[13]: True
+       ...: 
+      File "<ipython-input-1-727bb88eaf33>", line 1
+        [ins] In [13]: 2 * 2 == 4
+              ^
+    SyntaxError: invalid syntax
+
+This is now fixed, and Vi prompt prefixes - ``[ins]`` and ``[nav]`` -  are
+skipped just as the normal ``In`` would be.
+
+IPython shell can be started in the Vi mode using ``ipython
+--TerminalInteractiveShell.editing_mode=vi``