feat(edit): insert an unsimplified key using CTRL-SHIFT-V

This marks the following Vim patches as ported:

vim-patch:8.1.2333: with modifyOtherKeys CTRL-^ doesn't work

Problem:    With modifyOtherKeys CTRL-^ doesn't work.
Solution:   Handle the exception.
828ffd5963

vim-patch:8.1.2350: other text for CTRL-V in Insert mode with modifyOtherKeys

Problem:    Other text for CTRL-V in Insert mode with modifyOtherKeys.
Solution:   Convert the Escape sequence back to key as if modifyOtherKeys is
            not set, and use CTRL-SHIFT-V to get the Escape sequence itself.
            (closes vim/vim#5254)
fc4ea2a72d

vim-patch:8.2.2084: CTRL-V U doesn't work to enter a Unicode character

Problem:    CTRL-V U doesn't work to enter a Unicode character when
            modifyOtherKeys is effective. (Ken Takata)
Solution:   Add a flag to get_literal() for the shift key. (closes vim/vim#7413)
0684e36a7e

Omit getcmdkeycmd() change as it depends on Vim patch 8.2.2062, which
may introduce a potential breakage.

test/functional/editor/mode_insert_spec.lua

@@ -131,6 +131,11 @@ describe('insert-mode', function()
     end)
   end)
 
+  it('Ctrl-Shift-V supports entering unsimplified key notations', function()
+    feed('i<C-S-V><C-J><C-S-V><C-@><C-S-V><C-[><C-S-V><C-S-M><C-S-V><M-C-I><C-S-V><C-D-J><Esc>')
+    expect('<C-J><C-@><C-[><C-S-M><M-C-I><C-D-J>')
+  end)
+
   describe([[With 'insertmode', Insert mode is not re-entered immediately after <C-L>]], function()
     before_each(function()
       command('set insertmode')