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/inheritance/is.wren

@@ -11,12 +11,12 @@ var a = A.new()
 var b = B.new()
 var c = C.new()
 
-IO.print(a is A) // expect: true
-IO.print(a is B) // expect: false
-IO.print(a is C) // expect: false
-IO.print(b is A) // expect: true
-IO.print(b is B) // expect: true
-IO.print(b is C) // expect: false
-IO.print(c is A) // expect: true
-IO.print(c is B) // expect: true
-IO.print(c is C) // expect: true
+System.print(a is A) // expect: true
+System.print(a is B) // expect: false
+System.print(a is C) // expect: false
+System.print(b is A) // expect: true
+System.print(b is B) // expect: true
+System.print(b is C) // expect: false
+System.print(c is A) // expect: true
+System.print(c is B) // expect: true
+System.print(c is C) // expect: true