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/map/remove.wren

@@ -4,15 +4,15 @@ var map = {
   "three": 3
 }
 
-IO.print(map.count) // expect: 3
-IO.print(map.remove("two")) // expect: 2
-IO.print(map.count) // expect: 2
-IO.print(map.remove("three")) // expect: 3
-IO.print(map.count) // expect: 1
+System.print(map.count) // expect: 3
+System.print(map.remove("two")) // expect: 2
+System.print(map.count) // expect: 2
+System.print(map.remove("three")) // expect: 3
+System.print(map.count) // expect: 1
 
 // Remove an already removed entry.
-IO.print(map.remove("two")) // expect: null
-IO.print(map.count) // expect: 1
+System.print(map.remove("two")) // expect: null
+System.print(map.count) // expect: 1
 
-IO.print(map.remove("one")) // expect: 1
-IO.print(map.count) // expect: 0
+System.print(map.remove("one")) // expect: 1
+System.print(map.count) // expect: 0