add example showcasing how to force wasm to run in the background tab

[cBournhonesque]

Objective

Fixes #13368

Solution

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Testing

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Changelog

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• What changed as a result of this PR?
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  • If you can't summarize the work, your change may be unreasonably large / unrelated. Consider splitting your PR to make it easier to review and merge!

Migration Guide

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• If this PR is a breaking change (relative to the last release of Bevy), describe how a user might need to migrate their code to support these changes
• Simply adding new functionality is not a breaking change.
• Fixing behavior that was definitely a bug, rather than a questionable design choice is not a breaking change.

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.