Support single line comments of the form // comment (#2254)

* Add support for single line comments of the form //

* Fix edge case where // inside a sequence wrapped in braces didn't break.

```reason
let x = {
  //
};
```

If the { }-sequence didn't break we would get parse error:

```reason
let x = { // };
```

Author: IwanKaramazow

formatTest/unit_tests/expected_output/basicStructures.re

@@ -147,7 +147,7 @@ let x = arr^[0] = 1;
  *============================================================================
  */;
 
-let (/\/) = (+); /* // is not a comment */
+let (/++) = (+); /* // indicates the start of a comment, not an infix op */
 
 let something =
   if (self.ext.logSuccess) {

formatTest/unit_tests/expected_output/emptyFileComment.re

@@ -0,0 +1 @@
+// file with just a single line comment