V4: Message builder

[quinchs]

Summary

This PR adds a new builder called MessageBuilder which can build a Message that is used in SendMessageAsync functions.
The upsides to this style is we can merge SendFile and SendMessage functions as the library can determine whether or not the message contains files.
The builder also makes building reusable message templates easier.

Todo

• IMessageChannel.SendMessageAsync
• Interaction responses
• Module base methods

[d4n3436]

There should be some abstraction layer for MessageBuilder and Message like interfaces, so people can create their own custom builders.

[quinchs]

We could just make stuff protected and virtual?

[csmir]

@quinchs thoughts on allowing message parameters to be provided with a string operator?

    public class Message
    {
        private Message(string input)
        {

        }
        
        public static implicit operator Message(string input)
            => new Message(input);
    }

    internal class Example
    {
        public void ExampleMethod()
        {
            Send("test");

            void Send(Message message)
            {

            }
        }
    }

[csmir]

Also with the same design as above, an embed could serve as operator for message:

        private Message(Embed input)
        {

        }

        public static implicit operator Message(Embed embed)
            => new Message(embed);

[csmir]

With the mention of 'format', another idea to consider is a Format method, much like string.Format which runs over all entries of the builder looking for {} format entries and params object[] values as part of the Format method in these places. If not enough values are provided, escape and leave other format entries sit, or allow a bool to clear other format notations out of the string.

[quinchs]

@Rozen4334 lgtm