Fixed #5205: Ctrl+Alt+2 doesn't send ^[^@ (#5272)

## Summary of the Pull Request

Fixes #5205, by replacing another use of `MapVirtualKeyW` with `ToUnicodeEx`.
The latter just seems to be much more consistent at translating key combinations in general.
In this particular case though it fixes the issue, because there's no differentiation in `MapVirtualKeyW` for whether it failed to return a character (`'\0'`) or succeeded in turning `^@` into `'\0'`.
`ToUnicodeEx` on the other hand returns the success state separately from the translated character.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist
* [x] Closes #5205
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Tests added/passed
* [ ] Requires documentation to be updated
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #5205

## Detailed Description of the Pull Request / Additional comments

This PR changes the behavior of the `Ctrl+Alt+Key` handling slightly:
⚠️ `ToUnicodeEx` returns unshifted characters. ⚠️
For instance `Ctrl+Alt+a` is now turned into `^[^a`. Due to how ASCII works this is essentially the same though because `'A' & 0b11111` and `'a' & 0b11111` are the same.

## Validation Steps Performed

* Run `showkey -a`
* Ensured `Ctrl+Alt+Space` as well as `Ctrl+Alt+Shift+2` are turned into `^[^@`
* Ensured other, random `Ctrl+Alt+Key` combination behave identical to the current master

.github/actions/spelling/dictionary/microsoft.txt

@@ -39,6 +39,7 @@ systemroot
 taskkill
 tasklist
 tdbuildteamid
+VCRT
 vcruntime
 visualstudio
 VSTHRD
@@ -47,3 +48,4 @@ wslpath
 wtl
 wtt
 wttlog
+Xamarin

src/terminal/input/terminalInput.cpp

@@ -565,25 +565,33 @@ bool TerminalInput::HandleKey(const IInputEvent* const pInEvent)
     // for the 5 least significant ones to be zeroed out.
     if (keyEvent.IsAltPressed() && keyEvent.IsCtrlPressed())
     {
-        auto ch = keyEvent.GetCharData();
-        if (ch == UNICODE_NULL)
-        {
-            // For Alt+Ctrl+Key messages GetCharData() returns 0.
-            // The values of the ASCII characters and virtual key codes
-            // of <Space>, A-Z (as used below) are numerically identical.
-            // -> Get the char from the virtual key.
-            ch = keyEvent.GetVirtualKeyCode();
-        }
+        const auto ch = keyEvent.GetCharData();
+        const auto vkey = keyEvent.GetVirtualKeyCode();
+
+        // For Alt+Ctrl+Key messages GetCharData() usually returns 0.
+        // Luckily the numerical values of the ASCII characters and virtual key codes
+        // of <Space> and A-Z, as used below, are numerically identical.
+        // -> Get the char from the virtual key if it's 0.
+        const auto ctrlAltChar = keyEvent.GetCharData() != 0 ? keyEvent.GetCharData() : keyEvent.GetVirtualKeyCode();
+
         // Alt+Ctrl acts as a substitute for AltGr on Windows.
         // For instance using a German keyboard both AltGr+< and Alt+Ctrl+< produce a | (pipe) character.
         // The below condition primitively ensures that we allow all common Alt+Ctrl combinations
         // while preserving most of the functionality of Alt+Ctrl as a substitute for AltGr.
-        if (ch == UNICODE_SPACE || (ch > 0x40 && ch <= 0x5A))
+        if (ctrlAltChar == UNICODE_SPACE || (ctrlAltChar > 0x40 && ctrlAltChar <= 0x5A))
         {
             // Pressing the control key causes all bits but the 5 least
             // significant ones to be zeroed out (when using ASCII).
-            ch &= 0b11111;
-            _SendEscapedInputSequence(ch);
+            _SendEscapedInputSequence(ctrlAltChar & 0b11111);
+            return true;
+        }
+
+        // Currently, when we're called with Alt+Ctrl+@, ch will be 0, since Ctrl+@ equals a null byte.
+        // VkKeyScanW(0) in turn returns the vkey for the null character (ASCII @).
+        // -> Use the vkey to determine if Ctrl+@ is being pressed and produce ^[^@.
+        if (ch == UNICODE_NULL && vkey == LOBYTE(VkKeyScanW(0)))
+        {
+            _SendEscapedInputSequence(L'\0');
             return true;
         }
     }