feat(editor)!: insert-mode ctrl-r should work like paste #35477

Problem:
insert-mode ctrl-r input is treated like raw user input, which is almost
never useful. This means any newlines in the input are affected by
autoindent, etc., which is:
- slow
- usually breaks the formatting of the input

Solution:
- ctrl-r should be treated like a paste, not user-input.
- does not affect `<c-r>=`, so `<c-r>=@x` can still be used to get the
  old behavior.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

test/functional/editor/mode_insert_spec.lua

@@ -83,6 +83,36 @@ describe('insert-mode', function()
         {5:-- INSERT --}                                      |
       ]])
     end)
+
+    it('inserts named registers literally', function()
+      local screen = Screen.new(50, 6)
+      -- regular text without special charecter command
+      command('let @a = "test"')
+      feed('i<C-R>a<ESC>')
+      screen:expect([[
+        tes^t                                              |
+        {1:~                                                 }|*4
+                                                          |
+      ]])
+
+      -- text with backspace character gets written literally by default
+      command('let @a = "test\\<C-H>"')
+      feed('cc<C-R>a<ESC>')
+      screen:expect([[
+        test{18:^^H}                                            |
+        {1:~                                                 }|*4
+                                                          |
+      ]])
+
+      -- =@reg<CR> can be used to get effect of keypress
+      command('let @a = "test\\<C-H>"')
+      feed('cc<C-R>=@a<CR><ESC>')
+      screen:expect([[
+        te^s                                               |
+        {1:~                                                 }|*4
+                                                          |
+      ]])
+    end)
   end)
 
   describe('Ctrl-O', function()