New icons (bold, italic, and numbered list) and Chromatic config changes

[jattasNI]

Pull Request

🤨 Rationale

Rick created these icons for #794 and reached out to me directly to integrate them into Nimble.

👩‍💻 Implementation

Followed the process in nimble-tokens CONTRIBUTING

• the default icon names generated from Rick's file names seemed good enough to me. Font Awesome calls their "B" and "I" icons "bold" and "italic" too.
• had to manually remove a fill from one of the icons so that severity colors would be applied. I asked Rick to make a similar change in Illustrator

Also noticed that the icon metadata file was only used for tests, so moved it to a tests location.

🧪 Testing

Manual inspection of Storybook.

Chromatic didn't initially detect any icon changes because TurboSnap didn't notice that any dependencies of the icon story had changed. I added Chromatic externals configuration to the GitHub Actions configuration to include icon and token source files in their diff calculation, as described in Chromatic docs.

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.