Mark FileResolver as @private so it's not part of the generated docs

FileResolver is mostly meant to be used internally, however initially I thought it might give some idea how urls/filenames are resolved by the API. Looking at the generated documentation now, and also https://github.com/adobe/asset-compute-example-workers/pull/29/files, I realize that it is misleading and clients may be calling directly before calling the Photoshop API.

I think the class should be marked with `@private` so the documentation is no longer generated.

Find the FileResolver class and check how its documentation is generated. Mark it with `@private`; done means the class no longer appears in the generated documentation.

Written by the indexing model from the issue text.