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/comments/block.wren

@@ -1,5 +1,5 @@
 // In middle of line.
-IO.print/* ... */(/* */"ok"/* */) // expect: ok
+System.print/* ... */(/* */"ok"/* */) // expect: ok
 
 // Nested.
-IO.print(/* in /* nest */ out */"ok") // expect: ok
+System.print(/* in /* nest */ out */"ok") // expect: ok

test/language/comments/block_at_eof.wren

@@ -1,2 +1,2 @@
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok
 /* comment */

test/language/comments/line_at_eof.wren

@@ -1,2 +1,2 @@
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok
 // comment

test/language/comments/unicode.wren

@@ -8,4 +8,4 @@
 
 // TODO: What about combining characters?
 
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok

test/language/comments/unterminated_block.wren

@@ -1,3 +1,3 @@
 // expect error line 3
-IO.print("nope") /*
+System.print("nope") /*
 oops

test/language/comments/unterminated_nested_block.wren

@@ -1,4 +1,4 @@
 // expect error line 4
-IO.print("nope") /* /* /*
+System.print("nope") /* /* /*
 */
 oops