Add support for auto-discovering initial content

[marcospassos]

Summary

This PR adds support for automatically discovering both initial and fallback content when using the useContent hook.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[pkg-pr-new]

Open in Stackblitz

npm i https://pkg.pr.new/@croct/plug-react@524

commit: 7926ff1