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/core/system/print.wren

@@ -0,0 +1,12 @@
+class Foo {
+  construct new() {}
+
+  toString { "Foo.toString" }
+}
+
+// Calls toString on argument.
+System.print(Foo.new()) // expect: Foo.toString
+
+// Returns the argument.
+System.print(System.print(1) == 1) // expect: 1
+                                   // expect: true