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/fiber/yield.wren

@@ -1,14 +1,14 @@
 var fiber = Fiber.new {
-  IO.print("fiber 1")
+  System.print("fiber 1")
   Fiber.yield()
-  IO.print("fiber 2")
+  System.print("fiber 2")
   Fiber.yield()
-  IO.print("fiber 3")
+  System.print("fiber 3")
 }
 
 fiber.call()          // expect: fiber 1
-IO.print("main 1")    // expect: main 1
+System.print("main 1")    // expect: main 1
 fiber.call()          // expect: fiber 2
-IO.print("main 2")    // expect: main 2
+System.print("main 2")    // expect: main 2
 fiber.call()          // expect: fiber 3
-IO.print("main 3")    // expect: main 3
+System.print("main 3")    // expect: main 3