Summary

This PR further improves the Elide manifest editing experience in the IDE in a few ways:

• Custom module URIs in the form elide:project.pkl are now resolved correctly by the IDE and provide completion. These URIs were already supported by the CLI, but caused issues in the IDE as they are non-standard.
• Main class references in the jvm.main setting now link to the actual class and support go-to-declaration navigation, auto-completion of fully qualified class names, and error highlighting for unresolved references and invalid classes with no main method.
• Entrypoint script references in the entrypoint field now link to the project file if found, and provide completion options on editing.
• Integration with the Pkl plugin is now optional; manifest editing features will only be enabled if the Pkl plugin is present, but the Elide plugin no longer requires the Pkl plugin to be installed.