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/truncate.wren

@@ -1,13 +1,13 @@
-IO.print(123.truncate)      // expect: 123
-IO.print((-123).truncate)   // expect: -123
-IO.print(0.truncate)        // expect: 0
-IO.print((-0).truncate)     // expect: -0
-IO.print(0.123.truncate)    // expect: 0
-IO.print(12.3.truncate)     // expect: 12
-IO.print((-0.123).truncate) // expect: -0
-IO.print((-12.3).truncate)  // expect: -12
+System.print(123.truncate)      // expect: 123
+System.print((-123).truncate)   // expect: -123
+System.print(0.truncate)        // expect: 0
+System.print((-0).truncate)     // expect: -0
+System.print(0.123.truncate)    // expect: 0
+System.print(12.3.truncate)     // expect: 12
+System.print((-0.123).truncate) // expect: -0
+System.print((-12.3).truncate)  // expect: -12
 
 // Using 32-bit representation, values "beyond" those  two will lead to
 // approximation.
-IO.print((12345678901234.5).truncate)   // expect: 12345678901234
-IO.print((-12345678901234.5).truncate)  // expect: -12345678901234
+System.print((12345678901234.5).truncate)   // expect: 12345678901234
+System.print((-12345678901234.5).truncate)  // expect: -12345678901234