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/comparison.wren

@@ -1,27 +1,27 @@
-IO.print(1 < 2)    // expect: true
-IO.print(2 < 2)    // expect: false
-IO.print(2 < 1)    // expect: false
+System.print(1 < 2)    // expect: true
+System.print(2 < 2)    // expect: false
+System.print(2 < 1)    // expect: false
 
-IO.print(1 <= 2)    // expect: true
-IO.print(2 <= 2)    // expect: true
-IO.print(2 <= 1)    // expect: false
+System.print(1 <= 2)    // expect: true
+System.print(2 <= 2)    // expect: true
+System.print(2 <= 1)    // expect: false
 
-IO.print(1 > 2)    // expect: false
-IO.print(2 > 2)    // expect: false
-IO.print(2 > 1)    // expect: true
+System.print(1 > 2)    // expect: false
+System.print(2 > 2)    // expect: false
+System.print(2 > 1)    // expect: true
 
-IO.print(1 >= 2)    // expect: false
-IO.print(2 >= 2)    // expect: true
-IO.print(2 >= 1)    // expect: true
+System.print(1 >= 2)    // expect: false
+System.print(2 >= 2)    // expect: true
+System.print(2 >= 1)    // expect: true
 
 // Zero and negative zero compare the same.
-IO.print(0 < -0) // expect: false
-IO.print(-0 < 0) // expect: false
-IO.print(0 > -0) // expect: false
-IO.print(-0 > 0) // expect: false
-IO.print(0 <= -0) // expect: true
-IO.print(-0 <= 0) // expect: true
-IO.print(0 >= -0) // expect: true
-IO.print(-0 >= 0) // expect: true
+System.print(0 < -0) // expect: false
+System.print(-0 < 0) // expect: false
+System.print(0 > -0) // expect: false
+System.print(-0 > 0) // expect: false
+System.print(0 <= -0) // expect: true
+System.print(-0 <= 0) // expect: true
+System.print(0 >= -0) // expect: true
+System.print(-0 >= 0) // expect: true
 
 // TODO: Wrong type for RHS.