Update config schema generator to understand new config definition macros

[stackotter]

Swift Bundler now uses macros to define its configuration structs. Introducing these greatly reduced the amount of repetitive code required to specify custom keys, overlay types, flattened types, etc. However, these changes also further broke the schema generator (which was already a bit broken by the custom Decodable implementations for types such as AppConfiguration.Dependency).

The generator's implementation can be found in Sources/schema-gen. You can use the schema_gen.sh script to run the generator with the required parameters supplied automatically. To output the schema to stdout instead of Bundler.schema.json, just run ./schema_gen.sh stdout.