Fix: Forth code misidentified as F#

The `.fs` file extension is commonly used for both F# and Forth.  Currently, only F# is detected.

GitHub currently contains about 48000 files with an `.fs` file extension, which are detected as F# or OCaml.  Of those, I believe 15000 files are actually in Forth.

This pull request adds an F# sample file, and .fs as a Forth extension.

Tested with:
http://github.com/tptb/sforth
http://github.com/nfz/asforth
http://github.com/vdorr/incforth
http://github.com/fsharp/fsharp
http://github.com/forthy42/gforth
http://github.com/jmptable/simcrum
http://github.com/JohnEarnest/Mako
http://github.com/fsharp/FSharp.Data
http://github.com/jgreene/Simple.Query

More examples of misidentified code at GitHub can be found with these searches:
http://github.com/search?q=extension%3A.fs+dup
http://github.com/search?q=extension%3A.fs+void+main