feat: User#flags (#4060)

* feat: user flags

* fix: unnecessary negated statement

* fix: wording for description

* fix: an vs. a

* feat: add verified bot and dev flags

Co-Authored-By: Vlad Frangu <kingdgrizzle@gmail.com>

* typings :verified bot and dev flags

Co-Authored-By: Vlad Frangu <kingdgrizzle@gmail.com>

* feat: mon's suggestion, async fetchFlags & jsdoc

* feat: added to index.js

* fix: typo

* style: leveled flags

* typings: update leveled flags

Co-authored-by: Vlad Frangu <kingdgrizzle@gmail.com>

src/index.js

@@ -28,6 +28,7 @@ module.exports = {
   SnowflakeUtil: require('./util/Snowflake'),
   Structures: require('./util/Structures'),
   SystemChannelFlags: require('./util/SystemChannelFlags'),
+  UserFlags: require('./util/UserFlags'),
   Util: Util,
   version: require('../package.json').version,