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/subscript_operators.wren

@@ -9,9 +9,9 @@ class Foo {
 }
 
 var foo = Foo.new()
-IO.print(foo["a"]) // expect: 1-subscript a
-IO.print(foo["a", "b"]) // expect: 2-subscript a b
-IO.print(foo["a", "b", "c"]) // expect: 3-subscript a b c
-IO.print(foo["a"] = "value") // expect: 1-subscript setter a = value
-IO.print(foo["a", "b"] = "value") // expect: 2-subscript setter a b = value
-IO.print(foo["a", "b", "c"] = "value") // expect: 3-subscript setter a b c = value
+System.print(foo["a"]) // expect: 1-subscript a
+System.print(foo["a", "b"]) // expect: 2-subscript a b
+System.print(foo["a", "b", "c"]) // expect: 3-subscript a b c
+System.print(foo["a"] = "value") // expect: 1-subscript setter a = value
+System.print(foo["a", "b"] = "value") // expect: 2-subscript setter a b = value
+System.print(foo["a", "b", "c"] = "value") // expect: 3-subscript setter a b c = value