Update storybook config to display USWDS images when deploying to a subdir

[rocketnova]

Ticket

#97

Changes

What was added, updated, or removed in this PR.

• Update regex for storybook string replacement to support deploying to a subdirectory

Context for reviewers

Testing instructions, background context, more in-depth details of the implementation, and anything else you'd like to call out or ask reviewers. Explain how the changes were verified.

I deployed #89 to WIC MT Demo Project in order to get our storybook deployed using github pages. I found an issue where the images weren't displaying properly and made a PR for it: navapbc/wic-mt-demo-project-eligibility-screener#168 This PR is a port of that one.

See also Slack discussion: https://nava.slack.com/archives/C03G1SWD9H7/p1669820652173809

I inspected the generated github action artifact and confirmed that we are currently correctly prepending the BASE_PATH to any relative paths in storybook that begin with url(/. However, USWDS also uses this syntax url("/ and our regex doesn't catch that.

Testing

Screenshots, GIF demos, code examples or output to help show the changes working as expected. ProTip: you can drag and drop or paste images into this textbox.

1. Go to this github action run, which builds storybook for this branch and deploys it to github pages
2. Download the generated artifact
3. Unzip the artifact
4. Grep the unzipped files and confirm that "/uswds/img does not appear and that "template-application-nextjs/uswds/img does appear