Update core_version_requirement; create composer.json.

Cumulative Drupal 9 Readiness Patches.

This MR applies the Drupal 9 compatibility patches from

https://www.drupal.org/project/node_save_redirect/issues/3148241#comment-13672394

and adds a valid composer.json to the repository.

It is useful to have an issue fork for Drupal 9 compatibility, rather than simply a patch, because:

In short, you can't use a patch to tell Composer that a Drupal 8 module is compatible with Drupal 9.

Source: https://www.mediacurrent.com/blog/how-fix-catch-22-problem-drupal-9-fixes-composer/

Adding Issue Fork as a Composer repository:

To use this issue fork as a transitional D9-compatible package in your project, modify your composer.json:

    "repositories": {
        "drupal/node_save_redirect": {
            "type": "vcs",
            "url": "https://git.drupalcode.org/issue/node_save_redirect-3148241"
        },
        "drupal": {
          "type": "composer",
          "url": "https://packages.drupal.org/8",
          "exclude": ["drupal/node_save_redirect"]
        },

Requiring the patched package from the Issue Fork:

Execute the following command in a terminal:

$ composer require drupal/node_save_redirect:dev-3148241-automated-drupal-9

or manually add the requirement to composer.json:

    "require": {
        "drupal/node_save_redirect": "dev-3148241-automated-drupal-9",

Credit due, mainly, to Benji Fisher via: https://www.drupal.org/project/views_url_alias/issues/3142138#comment-14177409

NOTE: Tested with Drupal 8. Transitional D8 / D9 module now passes Upgrade Status D9 compatibility tests.

NOTE: Not yet tested with Drupal 9. You can help the Drupal community by testing this! 👍

Closes #3148241