Don't allow entrypoint file to be excluded from Project

[kgartland-rstudio]

You can exclude the entrypoint file from the Configuration File in the UI under Project Files. Since the entrypoint is required, can we block that from being excluded?

I know we can't stop them from manually editing the Configuration File but it seems like we can guard against them breaking themselves from doing it in the extension.

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• Add a pre-flight check to ensure the entrypoint is included
• Add a warning in the sidebar when the entrypoint is not included in the selected configuration
• Disallow the Files API from removing the entrypoint
• Disable the checkbox in the Project Files view (handled in Disable entrypoint checkbox in Project Files view #2487)

[mmarchetti]

Also see #1503

[mmarchetti]

Files API should return a "reason" for these named files that mandates their inclusion.

[mmarchetti]

Also should have a preflight check for this.