feat(editor): handle new multibyte sequences in normal mode replacement

while the implementation is not tied to screen chars, it is a reasonable
expectation to support the same size. If nvim is able to display a
multibyte character, it will accept the same character as input,
including in normal mode commands like r{char}

test/functional/editor/mode_normal_spec.lua

@@ -9,6 +9,7 @@ local feed = n.feed
 local fn = n.fn
 local command = n.command
 local eq = t.eq
+local api = n.api
 
 describe('Normal mode', function()
   before_each(clear)
@@ -41,4 +42,23 @@ describe('Normal mode', function()
       attr_ids = {},
     })
   end)
+
+  it('replacing with ZWJ emoji sequences', function()
+    local screen = Screen.new(30, 8)
+    screen:attach()
+    api.nvim_buf_set_lines(0, 0, -1, true, { 'abcdefg' })
+    feed('05r🧑‍🌾') -- ZWJ
+    screen:expect([[
+      🧑‍🌾🧑‍🌾🧑‍🌾🧑‍🌾^🧑‍🌾fg                  |
+      {1:~                             }|*6
+                                    |
+    ]])
+
+    feed('2r🏳️‍⚧️') -- ZWJ and variant selectors
+    screen:expect([[
+      🧑‍🌾🧑‍🌾🧑‍🌾🧑‍🌾🏳️‍⚧️^🏳️‍⚧️g                 |
+      {1:~                             }|*6
+                                    |
+    ]])
+  end)
 end)