* Add embed builder extensions

People in #dotnet_discord-net suggested that this should be part of
the lib after I demonstrated it

* Move some extensions into EmbedBuilder [2]

Apparently git didn't like that previous commit

* Fix error with EmbedBuilderExtensions

A summary of issues which happened:
- Git decided to add an amend commit (I told it to quit?)
- VS Code thinks everything is an error so it wasn't helpful
- dotnet decided to think there was no error until I deleted all
  build outputs and rebuild

Sometimes I question my ability to use version control properly.

Allows null embeds to be passed into ModifyAsync