Add coffescript syntax highlighting

Imported lexer from Scintilla and added coffeescript filetype
> […] as it is how the keywords are set to the Scintilla lexer: by their numerical ID, which is the order there.

Actually I was wrong, the order in highlightingmappings has nothing to do with it, the ID is the explicit numeric ID in the list there.  So you can completely omit the 3rd set (ID=2).  See https://github.com/geany/geany/blob/master/src/highlightingmappings.h#L56