feat: add json schema (#952)

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<!-- 1. Explain WHAT the change is about -->

- Add a self-hosted json schema that will be referenced to
[schemastore](https://www.schemastore.org/json/), part of
[MET-798](https://linear.app/metatypedev/issue/MET-798/metatype-schema-for-ide-support).

<!-- 3. Explain HOW users should update their code -->

#### Migration notes

---

- [x] The change comes with new or modified tests
- [ ] Hard-to-understand functions have explanatory comments
- [ ] End-user documentation is updated to reflect the change


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Added comprehensive configuration schema validation for Metatype
system configuration files.
- Implemented JSON schema testing to ensure configuration integrity
across multiple YAML files.

- **Tests**
- Introduced new test suite for validating configuration schema using
Ajv JSON schema validator.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/tools/schema_test.ts

@@ -0,0 +1,40 @@
+// Copyright Metatype OÜ, licensed under the Mozilla Public License Version 2.0.
+// SPDX-License-Identifier: MPL-2.0
+
+// NOTE: https://github.com/ajv-validator/ajv-formats/issues/85
+import Ajv from "https://esm.sh/[email protected]";
+import addFormats from "https://esm.sh/[email protected]";
+
+import { parse } from "npm:yaml";
+import schema from "@local/tools/schema/metatype.json" with { type: "json" };
+import * as path from "@std/path";
+import { assert } from "@std/assert";
+import { Meta } from "test-utils/mod.ts";
+
+const files = [
+  "../metatype.yml",
+  "../../examples/metatype.yaml",
+  "../../examples/templates/deno/metatype.yaml",
+  "../../examples/templates/node/metatype.yaml",
+  "../../examples/templates/python/metatype.yaml",
+  "../metagen/typegraphs/sample/metatype.yml",
+  "../metagen/typegraphs/identities/metatype.yml",
+];
+
+Meta.test("Configuration schema", () => {
+  const ajv = new Ajv();
+
+  addFormats(ajv);
+
+  const validate = ajv.compile(schema);
+  const scriptDir = import.meta.dirname!;
+
+  for (const file of files) {
+    const relativePath = path.resolve(scriptDir, file);
+    const yaml = Deno.readTextFileSync(relativePath);
+    const parsed = parse(yaml);
+    const result = validate(parsed);
+
+    assert(result, `validation failed for '${file}'`);
+  }
+});