feat: add embed-only message ctor

[osirais]

This PR adds an embed-only constructor to dpp::message. Since message(snowflake channel_id, const embed& _embed) exists, it seemed natural and useful to have a version without the channel id.

// this is probably the most common reply on my bot
event.reply(dpp::message{}.add_embed(embed));

// now you could just do this
event.reply(dpp::message{embed});

// or this with the implicit conversion
event.reply(embed);

Code change checklist

• [x] I have ensured that all methods and functions are fully documented using doxygen style comments.
• [x] My code follows the coding style guide.
• [x] I tested that my change works before raising the PR.
• [x] I have ensured that I did not break any existing API calls.
• [x] I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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