chore(postcss): streamline postcss configuration and add `postcss-d…

…iscard-unused` plugin

- Removed all `postcss` plugins from the `.postcssrc` file as they are covered by default via ParcelJS bundler.
- Added the `postcss-discard-unused` plugin to the `.postcssrc` file.

Changes summary:
- Streamlined `postcss` configuration by removing redundant plugins covered by ParcelJS bundler.
- Added the `postcss-discard-unused` plugin to optimize CSS output by removing unused styles.

.postcssrc

@@ -1,12 +1,5 @@
 {
 	"plugins": {
-		"css-has-pseudo": true,
-		"postcss-selector-not": true,
-		"postcss-custom-properties": true,
-		"postcss-gap-properties": true,
-		"postcss-clamp": true,
-		"postcss-transform": {
-			"preserve": false
-		}
+		"postcss-discard-unused": {}
 	}
 }

package.json

@@ -64,6 +64,7 @@
 		"adm-zip": "^0.5.10",
 		"parcel": "^2.11.0",
 		"postcss": "^8.4.32",
+		"postcss-discard-unused": "^6.0.2",
 		"postcss-preset-env": "^9.3.0",
 		"postcss-transform": "^0.0.2",
 		"rimraf": "^5.0.5",