Add support for short hex color codes like #CCC (#2639)

[kojoru]

Summary of the Pull Request

This adds a few lines to support shorthand color hex codes like #ABC. They are treated as equivalent of #AABBCC

References

N/A

PR Checklist

• Closes Add support for short hex color codes (like #CCC) #2639
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed
• Doesn't require documentation to be updated as far as I understand
• 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: Add support for short hex color codes (like #CCC) #2639

Detailed Description of the Pull Request / Additional comments

The PR is only a few lines of code so should be self-explanatory.

Validation Steps Performed

I've changed src/cascadia/ut_app/JsonTests.cpp thanks to the fact that Cascadia actually has a white color that can be expressed this way. The new test passes on my machine.

I thought that change might be a better idea than adding a new test to https://github.com/microsoft/terminal/blob/master/src/types/ut_types/UtilsTests.cpp as discussed originally, if only for the reason of simplicity. I'll be happy to write an explicit test if that seems like a better idea to maintainers.

[miniksa]

Good enough to me. I'm fine with the test in this place.

[DHowett-MSFT]

this is clever! Making two copies of the character before we parse it...

[ghost]

🎉Windows Terminal Preview v0.5.2661.0 has been released which incorporates this pull request.:tada:

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