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/number/ceil.wren

@@ -1,8 +1,8 @@
-IO.print(123.ceil)      // expect: 123
-IO.print((-123).ceil)   // expect: -123
-IO.print(0.ceil)        // expect: 0
-IO.print((-0).ceil)     // expect: -0
-IO.print(0.123.ceil)    // expect: 1
-IO.print(12.3.ceil)     // expect: 13
-IO.print((-0.123).ceil) // expect: -0
-IO.print((-12.3).ceil)  // expect: -12
+System.print(123.ceil)      // expect: 123
+System.print((-123).ceil)   // expect: -123
+System.print(0.ceil)        // expect: 0
+System.print((-0).ceil)     // expect: -0
+System.print(0.123.ceil)    // expect: 1
+System.print(12.3.ceil)     // expect: 13
+System.print((-0.123).ceil) // expect: -0
+System.print((-12.3).ceil)  // expect: -12