Move Overlay styles to PVC

[simurai]

Description

This is part of #1342 and on top of #1401. It adds the Overlay styles from PCSS. There should be no visual changes.

Integration

Does this change require any updates to code in production?

Yes, the following lines can be removed on dotcom:

- @import '@primer/css/overlay/overlay.scss';

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews

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Latest commit: b14e23f

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[simurai]

I only commented out these primer-overlay styles. It seems to be "test styles", but I assume they can be removed again at some point?

Before	After

[simurai]

@keithamus Not sure what the status of #1648 is, but feel free to merge in these styles if you like. Or you can also leave them as a separate PR and we can merge them later.

[keithamus]

Let's get these into main and forgo merging into #1401 for now. I'll rebase #1401 after the fact, as Dialog also uses these styles.

[simurai]

Superseded by #1651 that is intended to be merged into main.