Add displayName to typings and test cases

[alanbixby]

It appears displayNames are enabled globally; as such many endpoints include a displayName proprerty however our library does not accurately reflect this.

I would strongly advise updating our unit tests in addition to our typings to avoid issues such as #372, where types were added but our code artificially excluded displayName.

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Resources such as jkmdev's API tracker should aid in this effort, using git grep displayName to show all additions of displayName, but we will still need to manually check all of the undocumented endpoints (economy, www, etc).

[Neztore]

Auth/Auth v1
Auth/Auth v2
Auth/Auth v2
Badges/Badges v1
Badges/Badges v1
Chat/Chat v2
Chat/Chat v2
Develop/Develop v1
Develop/Develop v1
Friends/Friends v1
Friends/Friends v1
Friends/Friends v1
Friends/Friends v1
Games/Games v1
Games/Games v1
Games/Games v1
Groups/Groups v1
Groups/Groups v1
Groups/Groups v2
Inventory/Inventory v1
Inventory/Inventory v1
Locale/Locale v1
PrivateMessages/PrivateMessages v1
Trades/Trades v1
Users/Users v1
Users/Users v1
Users/Users v1
Users/Users v1
Users/Users v1
Users/Users v1