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/edit.c

@@ -1587,7 +1587,8 @@ static void ins_ctrl_v(void)
 
   add_to_showcmd_c(Ctrl_V);
 
-  c = get_literal();
+  // Do not include modifiers into the key for CTRL-SHIFT-V.
+  c = get_literal(mod_mask & MOD_MASK_SHIFT);
   if (did_putchar) {
     // when the line fits in 'columns' the '^' is at the start of the next
     // line and will not removed by the redraw
@@ -5612,13 +5613,13 @@ static unsigned quote_meta(char_u *dest, char_u *src, int len)
   return m;
 }
 
-/*
- * Next character is interpreted literally.
- * A one, two or three digit decimal number is interpreted as its byte value.
- * If one or two digits are entered, the next character is given to vungetc().
- * For Unicode a character > 255 may be returned.
- */
-int get_literal(void)
+/// Next character is interpreted literally.
+/// A one, two or three digit decimal number is interpreted as its byte value.
+/// If one or two digits are entered, the next character is given to vungetc().
+/// For Unicode a character > 255 may be returned.
+///
+/// @param  no_simplify  do not include modifiers into the key
+int get_literal(bool no_simplify)
 {
   int cc;
   int nc;
@@ -5636,6 +5637,9 @@ int get_literal(void)
   i = 0;
   for (;;) {
     nc = plain_vgetc();
+    if (!no_simplify) {
+      nc = merge_modifiers(nc);
+    }
     if ((mod_mask & ~MOD_MASK_SHIFT) != 0) {
       // A character with non-Shift modifiers should not be a valid
       // character for i_CTRL-V_digit.

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

src/nvim/getchar.c

@@ -1442,6 +1442,27 @@ static void updatescript(int c)
   }
 }
 
+/// Merge "mod_mask" into "c_arg"
+int merge_modifiers(int c_arg)
+{
+  int c = c_arg;
+
+  if (mod_mask & MOD_MASK_CTRL) {
+    if ((c >= '`' && c <= 0x7f) || (c >= '@' && c <= '_')) {
+      c &= 0x1f;
+      mod_mask &= ~MOD_MASK_CTRL;
+      if (c == 0) {
+        c = K_ZERO;
+      }
+    } else if (c == '6') {
+      // CTRL-6 is equivalent to CTRL-^
+      c = 0x1e;
+      mod_mask &= ~MOD_MASK_CTRL;
+    }
+  }
+  return c;
+}
+
 /// Get the next input character.
 /// Can return a special key or a multi-byte character.
 /// Can return NUL when called recursively, use safe_vgetc() if that's not
@@ -1603,19 +1624,7 @@ int vgetc(void)
 
       // A modifier was not used for a mapping, apply it to ASCII
       // keys.  Shift would already have been applied.
-      if (mod_mask & MOD_MASK_CTRL) {
-        if ((c >= '`' && c <= 0x7f) || (c >= '@' && c <= '_')) {
-          c &= 0x1f;
-          mod_mask &= ~MOD_MASK_CTRL;
-          if (c == 0) {
-            c = K_ZERO;
-          }
-        } else if (c == '6') {
-          // CTRL-6 is equivalent to CTRL-^
-          c = 0x1e;
-          mod_mask &= ~MOD_MASK_CTRL;
-        }
-      }
+      c = merge_modifiers(c);
 
       // If mappings are enabled (i.e., not Ctrl-v) and the user directly typed
       // something with a meta- or alt- modifier that was not mapped, interpret

src/nvim/normal.c

@@ -5154,7 +5154,7 @@ static void nv_replace(cmdarg_T *cap)
   // get another character
   if (cap->nchar == Ctrl_V) {
     had_ctrl_v = Ctrl_V;
-    cap->nchar = get_literal();
+    cap->nchar = get_literal(false);
     // Don't redo a multibyte character with CTRL-V.
     if (cap->nchar > DEL) {
       had_ctrl_v = NUL;
@@ -5369,7 +5369,7 @@ static void nv_vreplace(cmdarg_T *cap)
       emsg(_(e_modifiable));
     } else {
       if (cap->extra_char == Ctrl_V) {          // get another character
-        cap->extra_char = get_literal();
+        cap->extra_char = get_literal(false);
       }
       stuffcharReadbuff(cap->extra_char);
       stuffcharReadbuff(ESC);