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/class/equality.wren

@@ -1,13 +1,13 @@
-IO.print(Num == Num)  // expect: true
-IO.print(Num == Bool) // expect: false
+System.print(Num == Num)  // expect: true
+System.print(Num == Bool) // expect: false
 
 // Not equal to other types.
-IO.print(Num == 123)  // expect: false
-IO.print(Num == true) // expect: false
+System.print(Num == 123)  // expect: false
+System.print(Num == true) // expect: false
 
-IO.print(Num != Num)  // expect: false
-IO.print(Num != Bool) // expect: true
+System.print(Num != Num)  // expect: false
+System.print(Num != Bool) // expect: true
 
 // Not equal to other types.
-IO.print(Num != 123)  // expect: true
-IO.print(Num != true) // expect: true
+System.print(Num != 123)  // expect: true
+System.print(Num != true) // expect: true