Fix close button not using the tab's text color (#8046) (#8209)

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## Summary of the Pull Request
This sets the tab close button color to match the tab text color.
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## References
#8046
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## PR Checklist
* [x] Closes #8046
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] 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: #8046

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## Detailed Description of the Pull Request / Additional comments
This sets the tab close button color to match the tab text color.
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## Validation Steps Performed
Test light theme white tab mouse hover effect:
![Screenshot_2020-11-07_152110_3](https://user-images.githubusercontent.com/7143383/98640319-ec735e80-22de-11eb-8d35-08188405b566.png)

Test dark theme white tab mouse hover effect:
![Screenshot_2020-11-07_153021_2](https://user-images.githubusercontent.com/7143383/98640377-f006e580-22de-11eb-9bb5-dde9fe9b81b6.png)

(cherry picked from commit 2a34080)

src/cascadia/TerminalApp/TerminalTab.cpp

@@ -855,6 +855,8 @@ namespace winrt::TerminalApp::implementation
         TabViewItem().Resources().Insert(winrt::box_value(L"TabViewItemHeaderForegroundPointerOver"), fontBrush);
         TabViewItem().Resources().Insert(winrt::box_value(L"TabViewItemHeaderForegroundPressed"), fontBrush);
         TabViewItem().Resources().Insert(winrt::box_value(L"TabViewButtonForegroundActiveTab"), fontBrush);
+        TabViewItem().Resources().Insert(winrt::box_value(L"TabViewButtonForegroundPressed"), fontBrush);
+        TabViewItem().Resources().Insert(winrt::box_value(L"TabViewButtonForegroundPointerOver"), fontBrush);
 
         _RefreshVisualState();