docs: add JSON Schema definitions for all message types (draft 2020-12)

C2S (7): send, join, part, nick, topic, mode, ping
S2C (12): message, dm, notice, join, part, quit, nick, topic, mode, system, error, pong
S2S (6): relay, link, unlink, sync, ping, pong

Each message type has its own schema file under schema/{c2s,s2c,s2s}/.
schema/README.md provides an index of all types with descriptions.

schema/s2c/error.json

@@ -0,0 +1,33 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/s2c/error.json",
+  "title": "S2C Error",
+  "description": "Error message delivered via the message stream.",
+  "type": "object",
+  "properties": {
+    "id": {
+      "type": "integer"
+    },
+    "type": {
+      "const": "error"
+    },
+    "ts": {
+      "type": "string",
+      "format": "date-time"
+    },
+    "code": {
+      "type": "string",
+      "description": "Machine-readable error code.",
+      "examples": ["nick_in_use", "no_such_channel", "not_on_channel", "permission_denied"]
+    },
+    "content": {
+      "type": "string",
+      "description": "Human-readable error message."
+    },
+    "channel": {
+      "type": "string",
+      "description": "Related channel, if applicable."
+    }
+  },
+  "required": ["id", "type", "ts", "code", "content"]
+}