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/sequence/map.wren

@@ -29,20 +29,20 @@ class Fib is Sequence {
 var squareFib = Fib.new().map {|fib| fib * fib }
 var iterator = null
 
-IO.print(squareFib is Sequence) // expect: true
-IO.print(squareFib) // expect: instance of MapSequence
+System.print(squareFib is Sequence) // expect: true
+System.print(squareFib) // expect: instance of MapSequence
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 0
+System.print(squareFib.iteratorValue(iterator)) // expect: 0
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 1
+System.print(squareFib.iteratorValue(iterator)) // expect: 1
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 1
+System.print(squareFib.iteratorValue(iterator)) // expect: 1
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 4
+System.print(squareFib.iteratorValue(iterator)) // expect: 4
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 9
+System.print(squareFib.iteratorValue(iterator)) // expect: 9