Add ASTNode.warning macro method

[Blacksmoke16]

Currently ASTNode.raise exists to raise a compile time exception focused on the given node.

I think it would be quite handy to have similar functionality for warnings. The use case being producing deprecation warnings within custom macro generation code that isn't able to use the annotation.

[Blacksmoke16]

Granted this and ASTNode.raise aren't as helpful as they could be due to #7147 and #7394.

Getting those fixed would make this much more helpful.