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/module/change_imported_value/change_imported_value.wren

@@ -1,11 +1,11 @@
 import "module" for Module, Other
 
-IO.print(Module) // expect: before
+System.print(Module) // expect: before
 
 // Reassigning the variable in the other module does not affect this one's
 // binding.
 Other.change
-IO.print(Module) // expect: before
+System.print(Module) // expect: before
 
 // But it does change there.
 Other.show // expect: after

test/language/module/change_imported_value/module.wren

@@ -7,6 +7,6 @@ class Other {
   }
 
   static show {
-    IO.print(Module)
+    System.print(Module)
   }
 }