set &define so things like ]d work for functions

For example `C-w d` jumps to the definition of the function under the cursor in
a split window, if it is defined in the same file or a file it sources.

Author: dag

ftplugin/fish.vim

@@ -1,5 +1,6 @@
 setlocal comments=:#
 setlocal commentstring=#%s
+setlocal define=\\v^\\s*function>
 setlocal foldexpr=fish#Fold()
 setlocal foldmethod=expr
 setlocal formatoptions+=ron1j