Rename PostCSS files to .pcss (#9013)

* Rename PostCSS files to `.pcss`

* Make Stylelint happy

* Delint

* Rename new files too

* delint

* Fix bad comment placement

package.json

@@ -20,7 +20,8 @@
     "CONTRIBUTING.rst",
     "LICENSE",
     "README.md",
-    "package.json"
+    "package.json",
+    ".stylelintrc.js"
   ],
   "main": "./src/index.ts",
   "matrix_src_main": "./src/index.ts",
@@ -47,7 +48,7 @@
     "lint:js": "eslint --max-warnings 0 src test cypress",
     "lint:js-fix": "eslint --fix src test cypress",
     "lint:types": "tsc --noEmit --jsx react && tsc --noEmit --jsx react -p cypress",
-    "lint:style": "stylelint \"res/css/**/*.scss\"",
+    "lint:style": "stylelint \"res/css/**/*.pcss\"",
     "test": "jest",
     "test:cypress": "cypress run",
     "test:cypress:open": "cypress open",
@@ -193,13 +194,14 @@
     "matrix-mock-request": "^2.0.0",
     "matrix-react-test-utils": "^0.2.3",
     "matrix-web-i18n": "^1.3.0",
+    "postcss-scss": "^4.0.4",
     "raw-loader": "^4.0.2",
     "react-test-renderer": "^17.0.2",
     "rimraf": "^3.0.2",
     "rrweb-snapshot": "1.1.7",
-    "stylelint": "^13.9.0",
-    "stylelint-config-standard": "^20.0.0",
-    "stylelint-scss": "^3.18.0",
+    "stylelint": "^14.9.1",
+    "stylelint-config-standard": "^26.0.0",
+    "stylelint-scss": "^4.2.0",
     "typescript": "^4.7.4",
     "walk": "^2.3.14"
   },