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/method/newlines.wren

@@ -7,14 +7,14 @@ class Foo {
 var foo = Foo.new()
 
 // Allow newlines after commas and before ")".
-IO.print(foo.method("a",
+System.print(foo.method("a",
 
     "b"
 
     )) // expect: method a b
 
 // Allow newlines after commas and before "]".
-IO.print(foo["a",
+System.print(foo["a",
 
     "b"