lua: complete-filename: use biggest possible prefix

This allows for example to complete file names in markdown images.

E.g.:    ![](pic<C-x><C-f>    will complete the file name.

Previously it would have detected the '[' as prefix.

lua/plugins/complete-filename.lua

@@ -13,7 +13,7 @@ local complete_filename = function(expand)
 	if not prefix then return end
 
 	-- Strip leading delimiters for some programming languages
-	local _, j = prefix:find("[{[(<'\"]+")
+	local _, j = prefix:find(".*[{[(<'\"]+")
 	if not expand and j then prefix = prefix:sub(j + 1) end
 
 	if prefix:match("^%s*$") then