Auto merge of #12330 - stshine:flexitem, r=pcwalton

Implement flexible box layout for row container

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This pull requests implements basic flexible box layout for row container.
It  has implemented most basic flexbox features, including grow, shrink, multi-line, *reverse properties, and alignment under `justify-content`, `align-items`, `align-self`, `align-content`.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X]  There are tests for these changes

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r? @pcwalton

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