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/c2s/part.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/c2s/part.json",
+  "title": "C2S Part",
+  "description": "Leave a channel. Submitted via DELETE /api/v1/channels/{name}.",
+  "type": "object",
+  "properties": {
+    "channel": {
+      "type": "string",
+      "description": "Channel name to leave.",
+      "pattern": "^#[a-zA-Z0-9_-]+$",
+      "examples": ["#general"]
+    },
+    "reason": {
+      "type": "string",
+      "description": "Optional part reason message.",
+      "maxLength": 256
+    }
+  },
+  "required": ["channel"],
+  "additionalProperties": false
+}