feat(validate_config): add warning

Author: stevenjoezhang

lib/hexo/validate_config.ts

@@ -27,8 +27,11 @@ export = (ctx: Hexo): void => {
   }
 
   if (!config.timezone) {
-    log.warn('No timezone setting detected! Using the default timezone UTC.');
-    config.timezone = 'UTC';
+    log.warn('No timezone setting detected! Using LocalTimeZone as the default timezone.');
+    log.warn('This behavior will be changed to UTC in the next major version. Please set timezone explicitly (e.g. LocalTimeZone or America/New_York) in _config.yml to avoid this warning.');
+    config.timezone = moment.tz.guess();
+  } else if (config.timezone.toLowerCase() === 'localtimezone') {
+    config.timezone = moment.tz.guess();
   } else {
     const configTimezone = moment.tz.zone(config.timezone);
     if (!configTimezone) {