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/abs.wren

@@ -1,6 +1,6 @@
-IO.print(123.abs)      // expect: 123
-IO.print((-123).abs)   // expect: 123
-IO.print(0.abs)        // expect: 0
-IO.print((-0).abs)     // expect: 0
-IO.print((-0.12).abs)  // expect: 0.12
-IO.print(12.34.abs)    // expect: 12.34
+System.print(123.abs)      // expect: 123
+System.print((-123).abs)   // expect: 123
+System.print(0.abs)        // expect: 0
+System.print((-0).abs)     // expect: 0
+System.print((-0.12).abs)  // expect: 0.12
+System.print(12.34.abs)    // expect: 12.34