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/setter/associativity.wren

@@ -13,6 +13,6 @@ var c = Foo.new("c")
 
 // Assignment is right-associative.
 a.bar = b.bar = c.bar = "d"
-IO.print(a.toString) // expect: d
-IO.print(b.toString) // expect: d
-IO.print(c.toString) // expect: d
+System.print(a.toString) // expect: d
+System.print(b.toString) // expect: d
+System.print(c.toString) // expect: d