vim ft: address the problem with broken highlighting in big files

It seems Vim needs to be instructed how to scan for the begining
of the multiline statements to correctly and efficiently highlight
lanuages.

See :help syn-sync-maxlines

pkgs/applications/editors/vim/ft-nix-support.patch

@@ -23,7 +23,7 @@ new file mode 100644
 index 0000000..a2f9918
 --- /dev/null
 +++ b/runtime/syntax/nix.vim
-@@ -0,0 +1,42 @@
+@@ -0,0 +1,47 @@
 +" Vim syntax file
 +" Language:	nix
 +" Maintainer:	Marc Weber <marco-oweber@gmx.de>
@@ -68,3 +68,8 @@ index 0000000..a2f9918
 +hi def link nixFuncArg     Identifier
 +
 +let b:current_syntax = "nix"
++
++" scan backwards to find begining of multiline statements
++syn sync ccomment nixMultiLineComment minlines=10 maxlines=500
++syn sync ccomment nixStringIndented minlines=10 maxlines=500
++syn sync ccomment nixString maxlines=10