Core library methods' source_location and in backtraces defined with Ruby code look like this:

• <internal:kernel>:89 on CRuby
• <internal:core> core/kernel.rb:533 on TruffleRuby
• uri:classloader:/jruby/kernel/enumerator.rb:243 on JRuby

I would suggest to use a <internal: prefix since this is becoming the de facto standard.
That will also provide a consistent way to filter out core library methods defined in Ruby when desirable.

Also note that both CRuby 3 and TruffleRuby ignore entries starting with <internal: for Kernel#warn(msg, uplevel: n) (details in ruby/rubygems#3987 (comment)).