[legacy-framework] jest-preset: handle static assets for next/image

[beerose]

Closes: blitz-js/legacy-framework#293

What are the changes and their implications?

Previously, when unit testing a component that uses next/image, we'd get an error that mocked test-file-stub string is not a valid argument — we were handing all types of static assets by replacing the import with test-file-stub. This PR handles files with extensions supported by next/image differently. Though, it's still possible to do manual mocks, and/or test real images. More context in the issue.

Open for feedback and alternative solutions.

Bug Checklist

• Integration test added (see test docs if needed)

Feature Checklist

• Integration test added (see test docs if needed)
• Documentation added/updated (submit PR to blitzjs.com repo)