Move Pathname to core and use mostly-Ruby version

* RubyPathname is stripped down to match the remaining extension
  in CRuby 3.5 (<=> and sub).
* RubyPathname.path field is bound to @path instance variable.
* pathname.rb comes from ruby/pathname, with undef preamble,
  extension load, and a few utility methods removed.
* stdlib pathname.rb contains those utility methods.

See ruby/pathname#57 and ruby/pathname#17

Fixes jruby#9060

Author: headius

core/src/main/java/org/jruby/Ruby.java

@@ -50,6 +50,7 @@
 import org.jruby.exceptions.LocalJumpError;
 import org.jruby.exceptions.SystemExit;
 import org.jruby.ext.jruby.JRubyUtilLibrary;
+import org.jruby.ext.pathname.RubyPathname;
 import org.jruby.ext.thread.ConditionVariable;
 import org.jruby.ext.thread.Mutex;
 import org.jruby.ext.thread.Queue;
@@ -477,6 +478,7 @@ private Ruby(RubyInstanceConfig config) {
         processModule = profile.allowModule("Process") ? RubyProcess.createProcessModule(context, objectClass, structClass) : null;
         timeClass = profile.allowClass("Time") ? RubyTime.createTimeClass(context, objectClass, comparableModule) : null;
         unboundMethodClass = profile.allowClass("UnboundMethod") ? RubyUnboundMethod.defineUnboundMethodClass(context, objectClass) : null;
+        pathnameClass = RubyPathname.createPathnameClass(context);
 
         if (profile.allowModule("Signal")) RubySignal.createSignal(context);
 
@@ -5253,6 +5255,7 @@ public interface RecursiveFunctionEx<T> extends ThreadContext.RecursiveFunctionE
     private final RubyClass closedQueueError;
     private final RubyClass sizedQueueClass;
     private final RubyClass dataClass;
+    private final RubyClass pathnameClass;
 
     private RubyClass tmsStruct;
     private RubyClass passwdStruct;