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/function/arity.wren

@@ -1,5 +1,5 @@
-IO.print(Fn.new {}.arity) // expect: 0
-IO.print(Fn.new {|a| a}.arity) // expect: 1
-IO.print(Fn.new {|a, b| a}.arity) // expect: 2
-IO.print(Fn.new {|a, b, c| a}.arity) // expect: 3
-IO.print(Fn.new {|a, b, c, d| a}.arity) // expect: 4
+System.print(Fn.new {}.arity) // expect: 0
+System.print(Fn.new {|a| a}.arity) // expect: 1
+System.print(Fn.new {|a, b| a}.arity) // expect: 2
+System.print(Fn.new {|a, b, c| a}.arity) // expect: 3
+System.print(Fn.new {|a, b, c, d| a}.arity) // expect: 4