* Improve parameter precondition type safety

Also removes some terrible code which was left over when I first
implemented parameter preconditions. I don't know why that was there.

With this commit, parameter preconditions should be much safer as they
use generic methods instead of janky casting of objects.

* Remove generic CheckPreconditions method

* Removed GuildUserExtensions and moved the methods to IGuildUser and implementations

* Made changes per fox's suggestion: Change->Modify. New Modify overload.

* Oops

* Per Volt: reimplemented new endpoints

* Fixing broken docstrings

* I forgot that docstrings are XML

* Implemented atomic add/remove role endpoints

* Removed  so people aren't irked

* Added single-item role add/remove methods

Fix detection of IDependencyMap impl

Added support for reaction 'me' field

UserTypeReader ArgumentNullException in DM

Automatic animated avatar detection

Fixed GetReactionsUsersAsync returning an empty enumerable

Update audio_ffmpeg.cs

Fixed the example precondition attribute

Add installing with nuget using VSC to guides

AudioApplication is a required argument

Thanks @MinnDevelopment for his awesome work on
the JDA guide that had no influence here at all.

🥄 better docs soon come

ConnectAsync -> StartAsync

This should resolve a plethora of relates issues, including user downloading not working (#542), possibly #531

A null conditional operator was required at line 70 to avoid a nullref.