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/assignment/local.wren

@@ -1,10 +1,10 @@
 Fn.new {
   var a = "before"
-  IO.print(a) // expect: before
+  System.print(a) // expect: before
 
   a = "after"
-  IO.print(a) // expect: after
+  System.print(a) // expect: after
 
-  IO.print(a = "arg") // expect: arg
-  IO.print(a) // expect: arg
+  System.print(a = "arg") // expect: arg
+  System.print(a) // expect: arg
 }.call()