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/equality.wren

@@ -1,19 +1,19 @@
-IO.print(123 == 123)  // expect: true
-IO.print(123 == 124)  // expect: false
-IO.print(-3 == 3)     // expect: false
-IO.print(0 == -0)     // expect: true
+System.print(123 == 123)  // expect: true
+System.print(123 == 124)  // expect: false
+System.print(-3 == 3)     // expect: false
+System.print(0 == -0)     // expect: true
 
 // Not equal to other types.
-IO.print(123 == "123") // expect: false
-IO.print(1 == true)    // expect: false
-IO.print(0 == false)   // expect: false
+System.print(123 == "123") // expect: false
+System.print(1 == true)    // expect: false
+System.print(0 == false)   // expect: false
 
-IO.print(123 != 123)  // expect: false
-IO.print(123 != 124)  // expect: true
-IO.print(-3 != 3)     // expect: true
-IO.print(0 != -0)     // expect: false
+System.print(123 != 123)  // expect: false
+System.print(123 != 124)  // expect: true
+System.print(-3 != 3)     // expect: true
+System.print(0 != -0)     // expect: false
 
 // Not equal to other types.
-IO.print(123 != "123") // expect: true
-IO.print(1 != true)    // expect: true
-IO.print(0 != false)   // expect: true
+System.print(123 != "123") // expect: true
+System.print(1 != true)    // expect: true
+System.print(0 != false)   // expect: true