feat(carbon-react): add Layer component

[joshblack]

Closes #9185

This PR adds in the Layer component and associated layer set tokens in @carbon/styles.

Changelog

New

• Add Layer component to @carbon/react

Changed

• Update scss/_theme.scss to include layer sets

Removed

Testing / Reviewing

• Verify tests pass as expected
• View the storybook story and make sure that the var(--bx-layer) token is changing as intended across each theme we support

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[emyarod]

looks good to me