refactor(logging): move the console line format into src/logging/format.ts

The printf template of config/logger.ts becomes formatLogLine, with the same
logic, so it can be unit tested without winston.

No behaviour change.

Refs libretech/mp#71

config/logger.ts

@@ -1,16 +1,10 @@
 import { winston } from "@strapi/logger";
+import { formatLogLine } from "../src/logging/format";
 
 export default {
 	transports: [
 		new winston.transports.Console({
-			format: winston.format.combine(
-				winston.format.printf(({ timestamp, level, message, ...rest }) => {
-					let restString = JSON.stringify(rest, undefined, 2);
-					restString = restString === "{}" ? "" : restString;
-
-					return `${timestamp}Z ${level}: ${message} ${restString}`;
-				})
-			)
+			format: winston.format.combine(winston.format.printf(formatLogLine))
 		})
 	]
 };