feat: Add hardware metadata schema.

* Initial hardware metadata JSON schema.
* GH Action to validate all schemas for boards/shields.

app/scripts/requirements.txt

@@ -0,0 +1,8 @@
+# Copyright (c) 2021 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+# Convert YAML to JSON for validation
+remarshal>=0.14.0
+
+# Perform our hardware metadata validation
+jsonschema>=3.2.0