Replace exports folder mappings with subpath patterns

Node.js 17 removed folder mappings (DEP0148): subpath keys that end in / in the exports and imports fields of package.json. Rewrites each one as the equivalent subpath pattern, so "./features/": "./src/features/" becomes "./features/*": "./src/features/*", including targets nested under conditions or in fallback arrays. Where the package already declares the pattern next to the folder mapping, the folder mapping is removed.