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/sqrt.wren

@@ -1,10 +1,10 @@
-IO.print(4.sqrt)        // expect: 2
-IO.print(1000000.sqrt)  // expect: 1000
-IO.print(1.sqrt)        // expect: 1
-IO.print((-0).sqrt)     // expect: -0
-IO.print(0.sqrt)        // expect: 0
-IO.print(2.sqrt)        // expect: 1.4142135623731
+System.print(4.sqrt)        // expect: 2
+System.print(1000000.sqrt)  // expect: 1000
+System.print(1.sqrt)        // expect: 1
+System.print((-0).sqrt)     // expect: -0
+System.print(0.sqrt)        // expect: 0
+System.print(2.sqrt)        // expect: 1.4142135623731
 
-IO.print((-4).sqrt.isNan) // expect: true
+System.print((-4).sqrt.isNan) // expect: true
 
 // TODO: Tests for sin and cos.