feat: set defaults for each picker in telescope setup (#883)

This allows easier picker configuration for example:

```lua
require("telescope").setup {
  pickers = {
    buffers = {
      show_all_buffers = true,
      sort_lastused = true,
      theme = "dropdown",
      previewer = false,
      mappings = {
        i = {
          ["<c-q>"] = "smart_send_to_qflist",
        }
      }
    }
  }
}
```

This configuration will be applied when running `:Telescope buffers`

.luacheckrc

@@ -19,6 +19,7 @@ globals = {
   "TelescopeCachedTails",
   "TelescopeCachedNgrams",
   "_TelescopeConfigurationValues",
+  "_TelescopeConfigurationPickers",
   "__TelescopeKeymapStore",
 }