Provide toString() in image objects created by lqip-loader

[tats-u]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Image objects created by lqip-loader don't provide their own toString(). It causes the [object Object] output when they are loaded via CSS (#10862).

Test Plan

• Confirm all images are loaded as usual.
• Should we add a component that has a background image in a dogfooding page?

Test links

Deploy preview: https://deploy-preview-10891--docusaurus-2.netlify.app

Related issues/PRs

#10862

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[slorber]

I didn't have time to investigate the linked issue yet, but this isn't the way I want to address this problem.

The loader could be disabled when the file is requested in a CSS file. We already do that for the SVGR plugin using issuer, because transforming SVGs to React component code doesn't make sense when using CSS such as url('/icon.svg'). This problem is quite similar.

[tats-u]

I see. Looks like it's less useful than I thought in lqip-loader.

[tats-u]

Note: still useful in responsive-loader.