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/range/to.wren

@@ -1,25 +1,25 @@
 // Ordered range.
-IO.print((2..5).to) // expect: 5
-IO.print((3..3).to) // expect: 3
-IO.print((0..3).to) // expect: 3
-IO.print((-5..3).to) // expect: 3
-IO.print((-5..-2).to) // expect: -2
+System.print((2..5).to) // expect: 5
+System.print((3..3).to) // expect: 3
+System.print((0..3).to) // expect: 3
+System.print((-5..3).to) // expect: 3
+System.print((-5..-2).to) // expect: -2
 
 // Backwards range.
-IO.print((5..2).to) // expect: 2
-IO.print((3..0).to) // expect: 0
-IO.print((3..-5).to) // expect: -5
-IO.print((-2..-5).to) // expect: -5
+System.print((5..2).to) // expect: 2
+System.print((3..0).to) // expect: 0
+System.print((3..-5).to) // expect: -5
+System.print((-2..-5).to) // expect: -5
 
 // Exclusive ordered range.
-IO.print((2...5).to) // expect: 5
-IO.print((3...3).to) // expect: 3
-IO.print((0...3).to) // expect: 3
-IO.print((-5...3).to) // expect: 3
-IO.print((-5...-2).to) // expect: -2
+System.print((2...5).to) // expect: 5
+System.print((3...3).to) // expect: 3
+System.print((0...3).to) // expect: 3
+System.print((-5...3).to) // expect: 3
+System.print((-5...-2).to) // expect: -2
 
 // Exclusive backwards range.
-IO.print((5...2).to) // expect: 2
-IO.print((3...0).to) // expect: 0
-IO.print((3...-5).to) // expect: -5
-IO.print((-2...-5).to) // expect: -5
+System.print((5...2).to) // expect: 2
+System.print((3...0).to) // expect: 0
+System.print((3...-5).to) // expect: -5
+System.print((-2...-5).to) // expect: -5