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.

src/nvim/ex_getln.c

@@ -2208,7 +2208,11 @@ static int command_line_handle_key(CommandLineState *s)
   case Ctrl_Q:
     s->ignore_drag_release = true;
     putcmdline('^', true);
-    s->c = get_literal();                 // get next (two) character(s)
+
+    // Get next (two) characters.
+    // Do not include modifiers into the key for CTRL-SHIFT-V.
+    s->c = get_literal(mod_mask & MOD_MASK_SHIFT);
+
     s->do_abbr = false;                   // don't do abbreviation now
     ccline.special_char = NUL;
     // may need to remove ^ when composing char was typed