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/SmartText.cs

@@ -30,7 +30,7 @@ public abstract record SmartText
             SmartPlainText spt => new SmartPlainText(spt.Text + input),
             SmartEmbedTextArray arr => arr with
             {
-                PlainText = arr.PlainText + input
+                Content = arr.Content + input
             },
             _ => throw new ArgumentOutOfRangeException(nameof(text))
         };
@@ -45,7 +45,7 @@ public abstract record SmartText
             SmartPlainText spt => new SmartPlainText(input + spt.Text),
             SmartEmbedTextArray arr => arr with
             {
-                PlainText = input + arr.PlainText
+                Content = input + arr.Content
             },
             _ => throw new ArgumentOutOfRangeException(nameof(text))
         };