Add warning macro

[Blacksmoke16]

Resolves #9246

[straight-shoota]

It would be nice to also check the location of the warning. But that can come as a future enhancement to assert_warning.

[Blacksmoke16]

While playing with the other macro error stuff I noticed that you can actually pass a variadic amount of arguments to macro raise? This keeps that behavior for consistency, but could also do a follow up to make it an error if are 0 or more than 1 arguments passed to #warning and add a deprecation to #raise.

$ crystal run --error-trace test.cr 
In test.cr:1:1

 1 | {% raise "foo", "bar", 123 %}
     ^
Error: foo bar 123