Move EmbedBuilder+Extensions to Discord.Net.Core

Previously it was implemented under Discord.Net.Rest, which seems
inconsistent and unnecessary.

This also allows Commands docstrings to reference EmbedBuilder, since
Commands only has a dependency on Core.

src/Discord.Net.Rest/AssemblyInfo.cs

@@ -1,7 +1,10 @@
-using System.Runtime.CompilerServices;
+using System.Runtime.CompilerServices;
 
 [assembly: InternalsVisibleTo("Discord.Net.Rpc")]
 [assembly: InternalsVisibleTo("Discord.Net.WebSocket")]
 [assembly: InternalsVisibleTo("Discord.Net.Webhook")]
 [assembly: InternalsVisibleTo("Discord.Net.Commands")]
-[assembly: InternalsVisibleTo("Discord.Net.Tests")]
+[assembly: InternalsVisibleTo("Discord.Net.Tests")]
+
+[assembly: TypeForwardedTo(typeof(Discord.EmbedBuilder))]
+[assembly: TypeForwardedTo(typeof(Discord.EmbedBuilderExtensions))]