Generate API support for declarative package.json

[lucas-koehler]

VSCode extensions allow specifying various properties and contributions via the extension's package.json.
The report does not consider this so far.

• VSCode extension manifest: https://code.visualstudio.com/api/references/extension-manifest
• VSCode extension manifest contribution points: https://code.visualstudio.com/api/references/contribution-points

Theia seems to (partially) implemented this in plugin-protocol.ts.

[colin-grant-work]

One possibility here would be to compare the definition of PluginPackage referred to above with VSCode's IRelaxedExtensionManifest in extennsions.ts. Assuming PluginPackage defines only fields and variants that are actually handled by Theia, any differences in the two packages would probably point to mismatches in expectations.