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/nonlocal/assignment.wren

@@ -1,7 +1,7 @@
 var Nonlocal = "before"
-IO.print(Nonlocal) // expect: before
+System.print(Nonlocal) // expect: before
 Nonlocal = "after"
-IO.print(Nonlocal) // expect: after
+System.print(Nonlocal) // expect: after
 
 class Foo {
   static method {
@@ -10,9 +10,9 @@ class Foo {
 }
 
 Foo.method
-IO.print(Nonlocal) // expect: method
+System.print(Nonlocal) // expect: method
 
 Fn.new {
   Nonlocal = "fn"
 }.call()
-IO.print(Nonlocal) // expect: fn
+System.print(Nonlocal) // expect: fn