Show available tokens tree in configuration form.

Starting work on it.

@msuthars, Thanks for the patch. There are some modifications that need to be done.
Please check the below reports.

FILE: ...8os/web/modules/contrib/notify_to_slack/notify_to_slack.info.yml
----------------------------------------------------------------------
FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
----------------------------------------------------------------------
 7 | WARNING | All dependencies must be prefixed with the project
   |         | name, for example "drupal:"
----------------------------------------------------------------------


FILE: ...os/web/modules/contrib/notify_to_slack/src/Form/SettingsForm.php
----------------------------------------------------------------------
FOUND 1 ERROR AND 4 WARNINGS AFFECTING 5 LINES
----------------------------------------------------------------------
   9 | ERROR   | [x] There must be one blank line after the last USE
     |         |     statement; 0 found;
  67 | WARNING | [ ] \Drupal calls should be avoided in classes, use
     |         |     dependency injection instead
  73 | WARNING | [ ] \Drupal calls should be avoided in classes, use
     |         |     dependency injection instead
 146 | WARNING | [ ] \Drupal calls should be avoided in classes, use
     |         |     dependency injection instead
 185 | WARNING | [ ] \Drupal calls should be avoided in classes, use
     |         |     dependency injection instead
----------------------------------------------------------------------
PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
----------------------------------------------------------------------

@msuthars and @naresh_bavaskar, I don't see any token module in drupal core. It is part as the utility. Update info file to contain token:token as dependency (Contrib Module). Let's create hook_update() to enable the token module. Remove the code to check token module is enabled or not in the Form.

Node, User and Site Information token types should be available.

@naresh_bavaskar in notify_to_slack.info.yml it should be `token:token` as the dependency.

@naresh_bavaskar, thanks for pointing out the issue. I have fixed it.