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/string/contains.wren

@@ -1,16 +1,16 @@
-IO.print("".contains(""))              // expect: true
-IO.print("anything".contains(""))      // expect: true
-IO.print("something".contains("meth")) // expect: true
-IO.print("something".contains("some")) // expect: true
-IO.print("something".contains("ing"))  // expect: true
-IO.print("something".contains("math")) // expect: false
+System.print("".contains(""))              // expect: true
+System.print("anything".contains(""))      // expect: true
+System.print("something".contains("meth")) // expect: true
+System.print("something".contains("some")) // expect: true
+System.print("something".contains("ing"))  // expect: true
+System.print("something".contains("math")) // expect: false
 
 // Non-ASCII.
-IO.print("søméthîng".contains("méth"))  // expect: true
-IO.print("søméthîng".contains("meth")) // expect: false
+System.print("søméthîng".contains("méth"))  // expect: true
+System.print("søméthîng".contains("meth")) // expect: false
 
 // 8-bit clean.
-IO.print("a\0b\0c".contains("\0")) // expect: true
-IO.print("a\0b\0c".contains("b")) // expect: true
-IO.print("a\0b\0c".contains("b\0c")) // expect: true
-IO.print("a\0b\0c".contains("bc")) // expect: false
+System.print("a\0b\0c".contains("\0")) // expect: true
+System.print("a\0b\0c".contains("b")) // expect: true
+System.print("a\0b\0c".contains("b\0c")) // expect: true
+System.print("a\0b\0c".contains("bc")) // expect: false