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 - https://github.com/github/linguist/commit/5a5d33499942d2a294ffec593539a9095f105882)