Update storybook config to display USWDS images when deploying to a subdir (#98)

## Ticket

#97

## Changes

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

## Context for reviewers

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](https://github.com/navapbc/template-application-nextjs/actions/runs/3633298101)
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

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

Co-authored-by: Sawyer Hollenshead <[email protected]>

Author: rocketnova

app/.storybook/main.js

@@ -49,8 +49,11 @@ const config = {
           options: {
             // Support deploying Storybook to a subdirectory (like GitHub Pages).
             // This adds the BASE_PATH to the beginning of all relative URLs in the CSS.
-            search: /url\(\//g,
-            replace: `url(${BASE_PATH}/`,
+            // It handles relative urls, whether they are quoted or not.
+            search: /url\(("?)\//g,
+            replace(match, p1, offset, string) {
+              return `url(${p1}${BASE_PATH}/`;
+            },
           },
         },
         {