feat(backgroundlayers): class based implementation

[jenndiaz]

Description

Creates utility classes which can be used by adding background layers to the dependencies for the component and then using the classes where needed.

Changes:

• Created a utility like "component" for background layers. This should eventually live in it's own section of the docs site. I didn't focus too much on navigation for the docs site or editing the nav.pug file.
• index.css has classes for each of the layers specified in the specs for background layers which can be used directly
• class names intentionally include the context it should be used in, editting vs browsing
• In storybook background layers is in it's own section "utility classes"

View Comments from previous branch for background layers that is now closed: https://github.com/adobe/spectrum-css/pull/2274

How and where has this been tested?

Please tag yourself on the tests you've marked complete to confirm the tests have been run by someone other than the author.

Validation steps

Regression testing

Validate:

1. The documentation pages for at least two other components are still loading, including:

• [ ] The pages render correctly, are accessible, and are responsive.

2. If components have been modified, VRTs have been run on this branch:

• [ ] VRTs have been run and looked at.
• [ ] Any VRT changes have been accepted (by reviewer and/or PR author), or there are no changes.

Screenshots

To-do list

• [ ] I have read the contribution guidelines.
• [ ] I have updated relevant storybook stories and templates.
• [ ] I have tested these changes in Windows High Contrast mode.
• [ ] If my change impacts other components, I have tested to make sure they don't break.
• [ ] If my change impacts documentation, I have updated the documentation accordingly.
• [ ] ✨ This pull request is ready to merge. ✨