[Suggestion] Get file JSON

We're able to create a file from a string of JSON but we can't get a file's JSON. This would be useful if combined with skript-json to set a variable to the string and it be serialized back into minecraft.

No file, test, or entry point is named. Start by tracing the existing path that creates a file from a JSON string, then determine how a file's JSON should be exposed for use with skript-json; done means the file can be read back as a serialized JSON string.

Written by the indexing model from the issue text.