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/sequence/all.wren

@@ -1,10 +1,10 @@
 var a = [1, 2, 3]
-IO.print(a.all {|x| x > 1 }) // expect: false
-IO.print(a.all {|x| x > 0 }) // expect: true
-IO.print([].all {|x| false }) // expect: true
+System.print(a.all {|x| x > 1 }) // expect: false
+System.print(a.all {|x| x > 0 }) // expect: true
+System.print([].all {|x| false }) // expect: true
 
 // Returns first falsey value.
-IO.print(a.all {|x| x < 2 ? null : false }) // expect: null
+System.print(a.all {|x| x < 2 ? null : false }) // expect: null
 
 // Returns last truthy value.
-IO.print(a.all {|x| x }) // expect: 3
+System.print(a.all {|x| x }) // expect: 3