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/mod.wren

@@ -1,17 +1,17 @@
-IO.print(5 % 3) // expect: 2
-IO.print(10 % 5) // expect: 0
-IO.print(-4 % 3) // expect: -1
-IO.print(4 % -3) // expect: 1
-IO.print(-4 % -3) // expect: -1
-IO.print(-4.2 % 3.1) // expect: -1.1
-IO.print(4.2 % -3.1) // expect: 1.1
-IO.print(-4.2 % -3.1) // expect: -1.1
+System.print(5 % 3) // expect: 2
+System.print(10 % 5) // expect: 0
+System.print(-4 % 3) // expect: -1
+System.print(4 % -3) // expect: 1
+System.print(-4 % -3) // expect: -1
+System.print(-4.2 % 3.1) // expect: -1.1
+System.print(4.2 % -3.1) // expect: 1.1
+System.print(-4.2 % -3.1) // expect: -1.1
 
 // Left associative.
-IO.print(13 % 7 % 4) // expect: 2
+System.print(13 % 7 % 4) // expect: 2
 
 // Precedence.
-IO.print(13 + 1 % 7) // expect: 14
+System.print(13 + 1 % 7) // expect: 14
 
 // TODO: Unsupported RHS types.
 // TODO: Error on mod by zero.