feat(preview): add Preview.is_active() function, closes #535

Author: cseickel

lua/neo-tree/sources/common/commands.lua

@@ -307,7 +307,6 @@ end
 -- END Git commands
 --------------------------------------------------------------------------------
 
-
 M.next_source = function(state)
   local sources = require("neo-tree").config.sources
   local next_source = sources[1]

lua/neo-tree/sources/common/preview.lua

@@ -346,6 +346,10 @@ Preview.hide = function()
   instance = nil
 end
 
+Preview.is_active = function()
+  return instance and instance.active
+end
+
 Preview.show = function(state)
   local node = state.tree:get_node()
   if node.type == "directory" then