Fixed #6377: TerminalCore::_altGrAliasing is undefined by default (#6571)

## Summary of the Pull Request

Fixes #6377. `TerminalCore` does not initialize `_altGrAliasing`. The impact is minimized in WT because it defaults to `true` in higher layers. It's not initialized when WPF is driving.

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## PR Checklist
* [x] Closes #6377
* [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
* [ ] 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: #xxx

src/cascadia/TerminalCore/Terminal.cpp

@@ -44,6 +44,7 @@ Terminal::Terminal() :
     _pfnWriteInput{ nullptr },
     _scrollOffset{ 0 },
     _snapOnInput{ true },
+    _altGrAliasing{ true },
     _blockSelection{ false },
     _selection{ std::nullopt }
 {