Rebol extension changes to languages.yml

Historically Rebol used the .r extension which unfortunately clashes with the R Stats programming language :(

For example even the Rebol interpreter repo says it uses 9.7% R instead of Rebol! - https://github.com/rebol/rebol

So 3 changes here…

1) .reb is now the primary (and official) extension - http://www.rebol.com/article/0540.html

2) .rebol moved to list of extensions (some code does use it)

3) .r added back.  NB. The majority of Rebol code on Rebol uses this (followed by .r2 & .r3).  .r was present in languages.yml previously but was removed for some reason? (looks like here - 5a5d334999)

lib/linguist/languages.yml

@@ -1494,10 +1494,12 @@ Rebol:
   type: programming
   lexer: REBOL
   color: "#358a5b"
-  primary_extension: .rebol
+  primary_extension: .reb
   extensions:
+  - .r
   - .r2
   - .r3
+  - .rebol
 
 Redcode:
   primary_extension: .cw