fix: update telescope extension structure (#72)

Telescope extension folder structure is described here:
https://github.com/nvim-telescope/telescope.nvim/wiki/Extensions#extension-folder-structure

When `telescope.load_extension(<name>)` is done, the name is loaded from
path `telescope._extensions.<name>`. With the current structure, one
could do `telescope.load_extension('finders')` and then it will be
available to load through `Telescope finders` which will error

> Error executing Lua callback: ...share/nvim/lazy/telescope.nvim/lua/telescope/command.lua:193: attempt to call a nil value
> stack traceback:
>     ...share/nvim/lazy/telescope.nvim/lua/telescope/command.lua:193: in function 'run_command'
>     ...share/nvim/lazy/telescope.nvim/lua/telescope/command.lua:253: in function 'load_command'
>     ...ocal/share/nvim/lazy/telescope.nvim/plugin/telescope.lua:108: in function <...ocal/share/nvim/lazy/telescope.nvim/plugin/telescope.lua:107>

lua/telescope/_extensions/persisted.lua

@@ -3,8 +3,8 @@ local pickers = require("telescope.pickers")
 local conf = require("telescope.config").values
 local action_state = require("telescope.actions.state")
 
-local _actions = require("telescope._extensions.actions")
-local _finders = require("telescope._extensions.finders")
+local _actions = require("telescope._extensions.persisted.actions")
+local _finders = require("telescope._extensions.persisted.finders")
 
 local telescope_opts = {}