Make panes use xaml star sizing (#4953)

## Summary of the Pull Request
Dustin and Mike had a discussion on star sizing. Mcpiroman connected the dots. I just swooped in and implemented it.

## References
#3996 (comment)
#3744 (comment)

## PR Checklist
* [X] Closes #3744

## Detailed Description of the Pull Request / Additional comments
Star sizing allows us to keep things proportional. Since a 1::1 proportion is the same as a 100::100 proportion, I just went in and added star sizing. In the words of a some dude with a big metal fist, everything is perfectly balanced now.

![image](https://user-images.githubusercontent.com/11050425/76813679-f7103f00-67b5-11ea-9b5c-d2cc73673aba.png)

## Validation Steps Performed
Verified for vertical, horizontal, and uneven splits.

(cherry picked from commit 1d8c5ba)

Author: carlos-zamora

src/cascadia/TerminalApp/Pane.cpp

@@ -803,10 +803,10 @@ void Pane::_CreateRowColDefinitions(const Size& rootSize)
         const auto paneSizes = _CalcChildrenSizes(rootSize.Width);
 
         auto firstColDef = Controls::ColumnDefinition();
-        firstColDef.Width(GridLengthHelper::FromPixels(paneSizes.first));
+        firstColDef.Width(GridLengthHelper::FromValueAndType(paneSizes.first, GridUnitType::Star));
 
         auto secondColDef = Controls::ColumnDefinition();
-        secondColDef.Width(GridLengthHelper::FromPixels(paneSizes.second));
+        secondColDef.Width(GridLengthHelper::FromValueAndType(paneSizes.second, GridUnitType::Star));
 
         _root.ColumnDefinitions().Append(firstColDef);
         _root.ColumnDefinitions().Append(secondColDef);
@@ -819,10 +819,10 @@ void Pane::_CreateRowColDefinitions(const Size& rootSize)
         const auto paneSizes = _CalcChildrenSizes(rootSize.Height);
 
         auto firstRowDef = Controls::RowDefinition();
-        firstRowDef.Height(GridLengthHelper::FromPixels(paneSizes.first));
+        firstRowDef.Height(GridLengthHelper::FromValueAndType(paneSizes.first, GridUnitType::Star));
 
         auto secondRowDef = Controls::RowDefinition();
-        secondRowDef.Height(GridLengthHelper::FromPixels(paneSizes.second));
+        secondRowDef.Height(GridLengthHelper::FromValueAndType(paneSizes.second, GridUnitType::Star));
 
         _root.RowDefinitions().Append(firstRowDef);
         _root.RowDefinitions().Append(secondRowDef);