fix: do not setup more than once

[FelisNivalis]

Description

related: #1628

Extensions, e.g. file_browser, merge default key mappings and user defined ones in their setups. the mappings contain actions which typically have __call and other metamethods.

However, when merging tables, vim.tbl_extend returns a table without a metatable. Thus vim.tbl_deep_extend({n = {g = action1}}, {n = {g = action2}}) will try to merge action1 and action2, but the returned table doesn't have metatables.

This is mainly an issue of vim.tbl_extend. I have opened an issue there, but it would be nice to also have things work without having to upgrade neovim.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Example config:

require('telescope').setup({
  extensions = {
    file_browser = {
      mappings = {n = {g = actions.close + my_action}}
    }
  }
})
telescope.load_extension("file_browser")

Trying to trigger the action will get an error: mappings.lua:290: attempt to call upvalue 'key_func' (a table value)

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)