Backport PR #10264: Indent on new line by looking at the text before the cursor.

that would allow edit like the following to insert the right amount of
space on the newline (`|` is the cursor)

    class Bar:
        def __init__(self):|<enter>
            self.filed=0

Instead of inserting a blank line with 0 space.

Closes  9283

---

 takluyver can you have a look ?

IPython/terminal/shortcuts.py

@@ -96,12 +96,14 def newline_or_execute_outer(shell):
                 b.cancel_completion()
             return
 
-        if not (d.on_last_line or d.cursor_position_row >= d.line_count
-            - d.empty_line_count_at_the_end()):
-            b.newline()
-            return
+        before_text = d.text[:d.cursor_position]
+        status, indent = shell.input_splitter.check_complete(before_text + '\n')
 
-        status, indent = shell.input_splitter.check_complete(d.text + '\n')
+        if not (d.on_last_line or
+                d.cursor_position_row >= d.line_count - d.empty_line_count_at_the_end()
+                ):
+            b.insert_text('\n' + (' ' * (indent or 0)))
+            return
 
         if (status != 'incomplete') and b.accept_action.is_returnable:
             b.accept_action.validate_and_handle(event.cli, b)