Full support for embed arrays in .greet/.bye, .say and other commands which use embeds

- Website to create them is live at eb2.nadeko.bot (it will soon be replacing eb.nadeko.bot)
- Embed arrays don't have a plainText property (it's renamed to 'content')
- Embed arrays use color hex values instead of an integer
- Old embed format will still work
- There shouldn't be any breaking changes

src/NadekoBot/Common/SmartText/SmartEmbedTextArray.cs

@@ -3,8 +3,8 @@ namespace NadekoBot;
 
 public sealed record SmartEmbedTextArray : SmartText
 {
-    public string PlainText { get; set; }
-    public SmartEmbedText[] Embeds { get; set; }
+    public string Content { get; set; }
+    public SmartEmbedArrayElementText[] Embeds { get; set; }
 
     public bool IsValid
         => Embeds?.All(x => x.IsValid) ?? false;