Tweak the Palette's KeyChord for High Contrast mode

[jtippet]

Summary of the Pull Request

Update the Palette to be readable under High Contrast mode

References

Regressed in #6833

PR Checklist

• Closes Command Palette key bindings are unreadable in High Contrast mode #6892
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed
• Documentation updated. If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated.
• 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: Command Palette key bindings are unreadable in High Contrast mode #6892

Detailed Description of the Pull Request / Additional comments

I pulled the styling of the KeyChord text into a Style, so we can give it a different style under High Contrast. Under HC, I just left all the colors at their default, so ListView can do its thing. (IMHO, the HC style now looks better than the non-HC mode, but maybe I'm biased ;) )

Validation Steps Performed

	Old	New
Light
Dark
HC White
HC Black

[DHowett]

Thank you for doing this. I also somewhat prefer how it looks without the lozenge. hmm

[ghost]

Hello @miniksa!

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[ghost]

🎉Windows Terminal Preview v1.2.2022.0 has been released which incorporates this pull request.:tada:

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