Returns a merged manifest object from expected files used to represent an application manifest: manifest.json, manifest.ts and manifest.js. If both a json and ts or js are present, then first the json file will be used as a base object, then the .ts or the .js file export will be merged over the json file. If a .ts file exists, the .js will be ignored. Otherwise, the .js file will be merged over the .json.

Retrieves a merged application manifest, validates the manifest and all its specified functions, and cleans up any bits from it not relevant for the Slack manifest APIs.