feat: Add NewContent into RoomMessageEvent

[diamondburned]

This commit adds the NewContent field into RoomMessageEvent. It is
supposed to be an object with the same type as RoomMessageEvent and
usually contains the message information that will replace the old
event's body.

For reference, here's an example JSON:

{
  "content": {
    "body": " * b",
    "m.new_content": {
      "body": "b",
      "msgtype": "m.text"
    },
    "m.relates_to": {
      "event_id": "$eventID",
      "rel_type": "m.replace"
    },
    "msgtype": "m.text"
  }
}

[chanbakjsd]

I think the comment above on m.relates_to should be updated.
It might also be worth noting in new_content's comment that it is part of MSC2676 (under review at matrix-org/matrix-spec-proposals#2676).

I am not sure what the best way to do this is but there should also be a way for the user to access RelType.

[diamondburned]

On second thought, since the user is manually adding the m.rel_type field
anyway, it would probably be better if they also manually add the NewContent
field by extending the struct.

Closing this PR because of that.