Write a JSON Schema file for subgraph.yaml #4322
Description
Improvement proposal
JSON Schema is a standard for specifying validation rules for JSON files and other file formats. In particular, it also happens to cover YAML, which is what we use for writing subgraph manifests.
JSON Schema alone is not sufficient for fully verifying a subgraph manifest (you need a graph-node instance for that), but it can provide a lot of support to subgraph developers. Editors can use JSON Schemas to display errors, autocompletion, and documentation when hovering on object properties. We will get all of these benefits if we add our JSON Schema file to https://github.com/SchemaStore/schemastore. Substreams already have one: SchemaStore/schemastore#2750.