Normalize :: syntax (#5048)

* normalize :: syntax

* fixes from code review

lib/coffeescript/grammar.js

@@ -603,6 +603,11 @@
       function() {
         return new Access(new PropertyName('prototype'));
       }),
+      o('?::',
+      function() {
+        return new Access(new PropertyName('prototype'),
+      'soak');
+      }),
       o('Index')
     ],
     // Indexing into an object or array using bracket notation.

lib/coffeescript/lexer.js

@@ -1634,7 +1634,7 @@
   // Other regexes.
   HERECOMMENT_ILLEGAL = /\*\//;
 
-  LINE_CONTINUER = /^\s*(?:,|\??\.(?![.\d])|::)/;
+  LINE_CONTINUER = /^\s*(?:,|\??\.(?![.\d])|\??::)/;
 
   STRING_INVALID_ESCAPE = /((?:^|[^\\])(?:\\\\)*)\\(?:(0[0-7]|[1-7])|(x(?![\da-fA-F]{2}).{0,2})|(u\{(?![\da-fA-F]{1,}\})[^}]*\}?)|(u(?!\{|[\da-fA-F]{4}).{0,4}))/; // Make sure the escape isn’t escaped.
   // octal escape