Added missing fibers transient dependency

[sdadn]

Summary

What was changed: This PR adds the missing fibers dependency which is a transient dependency of webpack-config-terra through sass-loader@10.

Why it was changed: Currently, the fibers dependency was installed through @wdio/sync in terra-functional-testing, causing an indirect dependency on that terra-functional-testing regardless of it is needed. Therefore, fibers was explicitly added to webpack-config-terra to decouple that link and to allow for removing of @wdio/sync for a future webdriverio upgrade.

Testing

This change was tested using:

• [ ] WDIO
• [ ] Jest
• [ ] Visual testing (please attach a screenshot or recording)
• [x] Other (please describe below)
• [ ] No tests are needed

This was tested by ensuring that the local dev-site works with npm start.

Reviews

In addition to engineering reviews, this PR needs:

• [ ] UX review
• [ ] Accessibility review
• [ ] Functional review

Additional Details

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Thank you for contributing to Terra. @cerner/terra

[sdadn]

All the tests are passing locally; the test failure is unrelated to this change.