fix(User): compare flags in #equals

[ImRodry]

Please describe the changes this PR makes and why it should be merged:
This PR compares the users' flags in User#equals. I had to create a new _equals method to be used in actions as these compare a user object with an API user, which does not have the same properties as a Discord.js User and thus would always return false when used this way. This wasn't necessarily needed as the user update action compares 2 User objects, but it's better than having broken code. Alternatively we can drop the checks on the GuildMemberUpdate and PresenceUpdate actions and this function won't be needed, let me know what you prefer

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating