[Bug] Fix dark mode on storybook web Docs pages

[narin]

Ticket #53

Description of Change

Quick fix to enable dark mode toggling on Docs pages. Dark mode was previously only working on the stories. This seems to be a known bug for Storybook. Found a workaround here. This also required renaming preview.ts to preview.tsx since it now includes React code.

Testing Packages

N/A (tested locally)

Screenshots/Video

https://github.com/department-of-veterans-affairs/va-mobile-library/assets/786704/88be8545-4019-4dfd-ad1c-52a0b22d3536

Testing

• [x] Tested on iOS
• [x] Tested on Android
• [x] Tested on Web

PR Checklist

Code reviewer validation:

• General
  • [x] PR is linked to ticket(s)
  • [x] PR has changelog label applied if it's to be included in the changelog
  • [x] Acceptance criteria:
    • All satisfied or
    • Documented reason for not being performed or
    • Split to separate ticket and ticket is linked by relevant AC(s)
  • [x] Above PR sections adequately filled out
  • [x] If any breaking changes, in accordance with the pre-1.0.0 versioning guidelines: a CU ticket has been created for the VA Mobile App detailing necessary adjustments with the package version that will be published by this ticket
• Code
  • [x] Tests are included if appropriate (or split to separate ticket)
  • [x] New functions have proper TSDoc annotations

Publish

If changes warrant a new version per the versioning guidelines and the PR is approved and ready to merge:

• [x] Merge main into branch
• [x] Merge branch to main
• [x] Publish new version after merging

[narin]

Fixed link. Couldn't connect it to the issue since it was closed already. ZenHub automatically connected the referenced storybook issue. Removed the connection.