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/language/list/grow_shrink.wren

@@ -8,4 +8,4 @@ for (i in 0..195) {
   list.removeAt(-1)
 }
 
-IO.print(list) // expect: [0, 1, 2, 3, 4]
+System.print(list) // expect: [0, 1, 2, 3, 4]

test/language/list/newlines.wren

@@ -6,19 +6,19 @@ var list = [
 
 ]
 
-IO.print(list[0]) // expect: a
-IO.print(list[1]) // expect: b
+System.print(list[0]) // expect: a
+System.print(list[1]) // expect: b
 
 // Newline after trailing comma.
 list = ["c",
 
 ]
 
-IO.print(list[0]) // expect: c
+System.print(list[0]) // expect: c
 
 // Newline in empty list.
 list = [
 
 ]
 
-IO.print(list.count) // expect: 0
+System.print(list.count) // expect: 0

test/language/list/trailing_comma.wren

@@ -1,4 +1,4 @@
 var list = ["a", "b",]
 
-IO.print(list[0]) // expect: a
-IO.print(list[1]) // expect: b
+System.print(list[0]) // expect: a
+System.print(list[1]) // expect: b