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/range/equality.wren

@@ -3,15 +3,15 @@ var b = 2..6
 var c = 2...5
 var d = 2...6
 
-IO.print(a == 2..5) // expect: true
-IO.print(a == 2..6) // expect: false
-IO.print(c == 2...5) // expect: true
-IO.print(c == 2...6) // expect: false
+System.print(a == 2..5) // expect: true
+System.print(a == 2..6) // expect: false
+System.print(c == 2...5) // expect: true
+System.print(c == 2...6) // expect: false
 
-IO.print(b != 2..5) // expect: true
-IO.print(b != 2..6) // expect: false
-IO.print(d != 2...5) // expect: true
-IO.print(d != 2...6) // expect: false
+System.print(b != 2..5) // expect: true
+System.print(b != 2..6) // expect: false
+System.print(d != 2...5) // expect: true
+System.print(d != 2...6) // expect: false
 
-IO.print(a != c) // expect: true
-IO.print(b != d) // expect: true
+System.print(a != c) // expect: true
+System.print(b != d) // expect: true