feat: add Sepia pixel shader for sepia tone effect

[AjaySinghAdhikari]

Summary of the Pull Request

References and Relevant Issues

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

PR Checklist

• Closes #xxx
• Tests added/passed
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)

[zadjii-msft]

Thanks for the submission!

I think I'm fine with us having a library of shader examples here, with the caveat that we're not really maintaining them. Of course, the better place for this would be somewhere like https://github.com/Hammster/windows-terminal-shaders - somewhere dedicated to showing these shaders off.

Before we accept this though, can you add a screenshot of this shader? Preferably one that includes the output of something like colortool, to show how it affects a range of colors?

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