Allow Vartheme BS4 theme to work with Composer ~2.0

Problem/Motivation

NOW that Drupal.org supports composer 2.0
As listed in Drupal Association Supporting Partner Update | September 2020

#3126566: Allow Drupal to work with Composer 2
#3128631: Update dependencies composer/composer ^2 and composer/semver to ^3

Allow varbase-project template to work with Composer ~2.0 #93
https://github.com/Vardot/varbase-project/issues/93

Proposed resolution

Change the following:

    "composer/installers": "~1.0",
    "oomphinc/composer-installers-extender": "~1.0",
    "cweagans/composer-patches": "~1.0",
    "drupal/drupal-library-installer-plugin": "^0.3",

To:

    "composer/installers": "~1.0 || ~2.0",
    "oomphinc/composer-installers-extender": "~1.0 || ~2.0",
    "cweagans/composer-patches": "~1.0",

Remove drupal/drupal-library-installer-plugin

User interface changes

None

API changes

Can use both Composer ~1.0 and Composer ~2.0

Data model changes

None