create JSON schema for slice definitions files

[cjdcordeiro]

Problem

The issue is depicted in #176. In short, we want to be able to validate new slice definitions that are being proposed, even though Chisel itself may ignore fields that are not part of the schema.

Suggested approach

Instead of making Chisel print warnings for unknown fields, the preferred approach is to create a manageable JSON schema within this repo, that can be updated alongside the code and used as a reference from the SchemaStore.

Pros:

• the repo itself will be the source of truth and hold the explicit schema that Chisel expects within a Slice Definitions File
  • with this JSON schema, our CI can do an early validation of new slice definitions without having to run Chisel
• adding this schema to the SchemaStore will allow contributors to have off-the-shelf validation within their IDEs

Additional considerations

When validating a slice definition, we may need to consider

1. the format version
2. the chisel-release
3. the chisel version

We need to carefully think about this because adding new fields doesn't equate to bumping the format.

Examples

Similar work has already been done for Rockcraft:

• Current JSON schema: https://github.com/canonical/rockcraft/blob/main/schema/rockcraft.json (original PR)
  • Rockcraft schema in the SchemaStore: https://github.com/SchemaStore/schemastore/blob/054e754b8c7318916a221bdd230a2c8b520330ca/src/api/json/catalog.json#L4486-L4491
• Existing Chisel slices schema in the SchemaStore (manually maintained): https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/chisel-slices.json