Add Ctrl+Backspace support (#3935)

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## Summary of the Pull Request
Changes the <kbd>Ctrl+Backspace</kbd> input sequence and how it is processed by `InputStateMachineEngine`. Now <kbd>Ctrl+Backspace</kbd> deletes a whole word at a time (tested on WSL, CMD, and PS).

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## References

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## PR Checklist
* [x] Closes #755
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed -> made minor edits to tests
* [ ] 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: #755

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## Detailed Description of the Pull Request / Additional comments
Changed the input sequence for <kbd>Ctrl+Backspace</kbd> to `\x1b\x8` so the sequence would pass through `_DoControlCharacter`. Changed `_DoControlCharacter` to process `\b` in a way which forms the correct `INPUT_RECORD`s to delete whole words.

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## Validation Steps Performed
<kbd>Ctrl+Backspace</kbd> works 🎉

(cherry picked from commit 2b79bd0)

src/terminal/parser/InputStateMachineEngine.cpp

@@ -204,7 +204,7 @@ bool InputStateMachineEngine::_DoControlCharacter(const wchar_t wch, const bool
     {
         // This is a C0 Control Character.
         // This should be translated as Ctrl+(wch+x40)
-        const wchar_t actualChar = wch;
+        wchar_t actualChar = wch;
         bool writeCtrl = true;
 
         short vkey = 0;
@@ -213,7 +213,9 @@ bool InputStateMachineEngine::_DoControlCharacter(const wchar_t wch, const bool
         switch (wch)
         {
         case L'\b':
-            success = _GenerateKeyFromChar(wch + 0x40, vkey, modifierState);
+            // Process Ctrl+Bksp to delete whole words
+            actualChar = '\x7f';
+            success = _GenerateKeyFromChar(actualChar, vkey, modifierState);
             modifierState = 0;
             break;
         case L'\r':

src/terminal/parser/ut_parser/InputEngineTest.cpp

@@ -349,6 +349,10 @@ void InputEngineTest::C0Test()
         case L'\t': // Tab
             writeCtrl = false;
             break;
+        case L'\b': // backspace
+            wch = '\x7f';
+            expectedWch = '\x7f';
+            break;
         }
 
         short keyscan = VkKeyScanW(expectedWch);