[Tag] Add secondary styling

[IlanaB]

This PR introduces a secondary variant of the Tag component. I'm looking for feedback and feasibility in merging this in advance of the Admin UI Foundations project

WHY are these changes introduced?

We're run into an issue where we have a component that contains both badges and tags and we'd like those to visually match. There's currently no way to achieve this without CSS overrides. Since we expect a secondary treatment to be coming I'm using this PR to test the waters in introducing this treatment now.

WHAT is this pull request doing?

This adds a "secondary" variation of the Tag that is compatible with all other variants and lightens the background to use bg-secondary as shown:

Note that on a secondary background the Tag background color will blend in

Currently includes:

• added new variant and matching styling
• added to storybook
• added to polaris docs

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

🎩 checklist

• Tested a snapshot
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide

[IlanaB]

@m-meurer @derekloshopify FYI!