feat(editor): add WebP support

[CoconutMacaroon]

Addresses MSE post: Allow WebP image uploads in the Stacks Editor

Describe your changes

Enables support for uploading WebP files (in addition to the existing support for JPEG/PNG/GIF)

PR Checklist

• All new/changed functionality includes unit and (optionally) e2e tests as appropriate
• All new/changed functions have /** ... */ docs
• I've added the bug/enhancement and other labels as appropriate

Environment(s) tested

• Device: Laptop
• OS: Windows 11 25H2
• Browser: Firefox
• Version: 156.0 (64-bit)

Additional context

I haven't added something like this for WebP, but I can (in test/shared/prosemirror-plugins/image-upload.test.ts)

await expect(
    uploader.showImagePreview(
        mockFile("some jpeg file", "image/jpeg")
    )
).resolves.toBeUndefined();
expect(findValidationMessage(uploader).classList).toContain(
    "d-none"
);

[changeset-bot]

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Latest commit: c15fbe5

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