feat(Guild): add systemChannelFlags (#3559)

* Add systemChannelFlags bitfield to Guild

* Implement @vladfrangu's suggestions

* fix: apply suggestions, reverse order of flags, reword docs

* docs: add SystemCHannelFlagsResolvable typedef

Co-authored-by: SpaceEEC <spaceeec@yahoo.com>

src/index.js

@@ -26,6 +26,7 @@ module.exports = {
   Snowflake: require('./util/Snowflake'),
   SnowflakeUtil: require('./util/Snowflake'),
   Structures: require('./util/Structures'),
+  SystemChannelFlags: require('./util/SystemChannelFlags'),
   Util: Util,
   util: Util,
   version: require('../package.json').version,