Drupal 9 Compatibility

We'll need to add Drupal 9 support for this module.

Here's the report from Upgrade Rector via the Upgrade Status module in Admin UI:

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Rector v0.6.13
Config file: ../../../../../private/tmp/upgrade_rector/upgrade_rector.yml

0/72 [░░░░░░░░░░░░░░░░░░░░░░░░░░░░] 0%
3/72 [▓░░░░░░░░░░░░░░░░░░░░░░░░░░░] 4%
8/72 [▓▓▓░░░░░░░░░░░░░░░░░░░░░░░░░] 11%
15/72 [▓▓▓▓▓░░░░░░░░░░░░░░░░░░░░░░░] 20%
22/72 [▓▓▓▓▓▓▓▓░░░░░░░░░░░░░░░░░░░░] 30%
72/72 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

[ERROR] Could not process
"/d8/web/modules/contrib/webpack/tests/src/Functional/LoadTest.php" file, due to:
"Analyze error: "Class PHPUnit\Framework\TestCase not found and could not be autoloaded.". Include your files
in "parameters > autoload_paths".
See https://github.com/rectorphp/rector#extra-autoloading".

[ERROR] Could not process
"/d8/web/modules/contrib/webpack/tests/src/Kernel/BundlerTest.php" file, due to:
"Analyze error: "Class PHPUnit\Framework\TestCase not found and could not be autoloaded.". Include your files
in "parameters > autoload_paths".
See https://github.com/rectorphp/rector#extra-autoloading".

[ERROR] Could not process
"/d8/web/modules/contrib/webpack/tests/src/Kernel/WebpackTestBase.php" file, due to:
"Analyze error: "Class PHPUnit\Framework\TestCase not found and could not be autoloaded.". Include your files
in "parameters > autoload_paths".
See https://github.com/rectorphp/rector#extra-autoloading".
---