Tools: Update packages (#451)

* Tools: Update packages

This updates the wp-env & wp-scripts packages, along with the configuration changes necessary to get the environment working. This includes phpunit updates, and removing the custom RTLCSS build config now that wp-scripts provides that.

* Add missing eslint plugin

* Fix spacing lint issue

.gitignore

@@ -29,3 +29,4 @@ phpcs.xml.dist
 !/source/wp-content/mu-plugins
 !/source/wp-content/mu-plugins/theme-switcher.php
 lighthouse.html
+source/wp-content/tests/phpunit/.phpunit.result.cache

.nvmrc

@@ -1 +1 @@
-14
+20

composer.json

@@ -109,21 +109,21 @@
 		"composer/installers": "~1.0",
 		"dealerdirect/phpcodesniffer-composer-installer": "^0.7.0",
 		"phpcompatibility/phpcompatibility-wp": "*",
-		"phpunit/phpunit": "^7.5.20",
+		"phpunit/phpunit": "9.6.*",
 		"wordpress-meta/pub": "1",
 		"wordpress-meta/wporg": "1",
 		"wordpress-meta/wporg-main": "1",
 		"wordpress-meta/handbook": "1",
 		"wordpress-meta/wporg-markdown": "1",
 		"wp-coding-standards/wpcs": "2.*",
-		"wp-phpunit/wp-phpunit": "^5.4",
+		"wp-phpunit/wp-phpunit": "6.*",
 		"wpackagist-plugin/gutenberg": "*",
 		"wpackagist-plugin/jetpack": "*",
 		"wpackagist-plugin/wordpress-importer": "*",
 		"wporg/wporg-repo-tools": "dev-trunk",
 		"wporg/wporg-mu-plugins": "dev-build",
 		"wporg/wporg-parent-2021": "dev-build",
-		"yoast/phpunit-polyfills": "^1.1"
+		"yoast/phpunit-polyfills": "2.*"
 	},
 	"scripts": {
 		"format": "phpcbf -p",