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)
Sample file for .reb Rebol extension
Sample file for .rebol Rebol extension
Sample file for .r2 Rebol extension
Sample file for .r3 Rebol extension
Sample file for .r Rebol extension
Disambiguate .r extension between Rebol and R
Samples for Rebol and R with the .r extension
Heuristics for Rebol and R
Regenerate samples