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/precedence.wren

@@ -1,41 +1,41 @@
 // * has higher precedence than +.
-IO.print(2 + 3 * 4) // expect: 14
+System.print(2 + 3 * 4) // expect: 14
 
 // * has higher precedence than -.
-IO.print(20 - 3 * 4) // expect: 8
+System.print(20 - 3 * 4) // expect: 8
 
 // / has higher precedence than +.
-IO.print(2 + 6 / 3) // expect: 4
+System.print(2 + 6 / 3) // expect: 4
 
 // / has higher precedence than -.
-IO.print(2 - 6 / 3) // expect: 0
+System.print(2 - 6 / 3) // expect: 0
 
 // < has higher precedence than ==.
-IO.print(false == 2 < 1) // expect: true
+System.print(false == 2 < 1) // expect: true
 
 // > has higher precedence than ==.
-IO.print(false == 1 > 2) // expect: true
+System.print(false == 1 > 2) // expect: true
 
 // <= has higher precedence than ==.
-IO.print(false == 2 <= 1) // expect: true
+System.print(false == 2 <= 1) // expect: true
 
 // >= has higher precedence than ==.
-IO.print(false == 1 >= 2) // expect: true
+System.print(false == 1 >= 2) // expect: true
 
 // is has higher precedence than ==.
-IO.print(true == 10 is Num) // expect: true
-IO.print(10 is Num == false) // expect: false
+System.print(true == 10 is Num) // expect: true
+System.print(10 is Num == false) // expect: false
 
 // Unary - has lower precedence than ..
-IO.print(-"abc".count) // expect: -3
+System.print(-"abc".count) // expect: -3
 
 // 1 - 1 is not space-sensitive.
-IO.print(1 - 1) // expect: 0
-IO.print(1 -1)  // expect: 0
-IO.print(1- 1)  // expect: 0
-IO.print(1-1)   // expect: 0
+System.print(1 - 1) // expect: 0
+System.print(1 -1)  // expect: 0
+System.print(1- 1)  // expect: 0
+System.print(1-1)   // expect: 0
 
 // TODO: %, associativity.
 
 // Using () for grouping.
-IO.print((2 * (6 - (2 + 2)))) // expect: 4
+System.print((2 * (6 - (2 + 2)))) // expect: 4