NextJS: Fix import url on windows

[nix6839]

Issue: @storybook/nextjs package failed on windows with following error:

ERR! Error [ERR_UNSUPPORTED_ESM_URL_SCHEME]: Only URLs with a scheme in: file, data are supported by the default ESM loader. On Windows, absolute paths must be valid file:// URLs. Received protocol 'c:'

What I did

Use the file:// scheme using pathToFileURL() when dynamic import.

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.