This changes manifest.preferences to a dictionary, which makes more sense:

1. Dictionary keys have the same constraints as what we need for the id (unique, non-empty strings), removing the need for some tests, validation and documentation.
2. Easier to declare/read (less code).
3. We can find the preference with preferences.get(id) instead of looping through all of them (slower and more code needed).

This change is fully backwards compatible. ExtensionManifest reformats the manifest as it loads it (this is something that I plan to let extension developers use to migrate extensions).

Checklist

• Verify that the test command ./ul test is passing (the CI server will check this if you don't)
• Update the documentation according to your changes (when applicable)
• Write unit tests for your changes (when applicable)