feat: replace IO class with System class and remove separate IO module

- Delete wren_io.c, wren_io.h, and io.wren source files
- Remove multi-arity print overloads and IO.read/IO.time methods
- Add System class with print, printAll, write, writeAll methods to core.wren
- Update all documentation examples and README to use System.print instead of IO.print

test/language/whitespace.wren

@@ -1,26 +1,26 @@
 // No indent
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok
 
 // Indent with space
-  IO.print("ok") // expect: ok
+  System.print("ok") // expect: ok
 
 // Indent with tab
-	IO.print("ok") // expect: ok
+	System.print("ok") // expect: ok
 
 // Indent with space then tab
-  	IO.print("ok") // expect: ok
+  	System.print("ok") // expect: ok
 
 // Indent with tab then space
-	  IO.print("ok") // expect: ok
+	  System.print("ok") // expect: ok
 
 // Indent with mixed tab and space
-  	  	IO.print("ok") // expect: ok
+  	  	System.print("ok") // expect: ok
 
 // Space in with code
-IO . print ( "ok" ) // expect: ok
+System . print ( "ok" ) // expect: ok
 
 // Tab in with code
-IO	.	print	(	"ok"	) // expect: ok
+System	.	print	(	"ok"	) // expect: ok
 
 // Tab and space mixed in with code
-IO	 . 	print 	(	 "ok" 	) // expect: ok
+System	 . 	print 	(	 "ok" 	) // expect: ok