Add Message.is_forwardable to check if a message can be forwarded

[n6ck]

Summary

Adds a new method Message.is_forwardable to check if a message can be forwarded with Message.forward
Helps to avoid failed forwarding attempts by providing a simple boolean check
Inspired by Message.is_system

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[pythonmcpi]

Is there an upstream docs page listing the criteria of whether a message is forwardable?

[Sacul0457]

given this is accepted, I think this should be in MessageType, not in Message. Similar to MessageType.is_deletable, this avoids implying that the message can be forwarded anywhere, regardless of permission.

But at the same time, you can't check for polls/activity etc, so...

Also, the attributes should never be missing afaik, you can just do if self.poll is not None instead of the getattr

[n6ck]

Is there an upstream docs page listing the criteria of whether a message is forwardable?

https://discord.com/developers/docs/resources/message#message-reference-types

[n6ck]

given this is accepted, I think this should be in MessageType, not in Message. Similar to MessageType.is_deletable, this avoids implying that the message can be forwarded anywhere, regardless of permission.

But at the same time, you can't check for polls/activity etc, so...

Also, the attributes should never be missing afaik, you can just do if self.poll is not None instead of the getattr

Good idea
As for getattr, self.call can be missing as I did come across that when testing for this, which is why I put that
(not even sure what self.call is tbh)