Rename PostCSS files to .pcss (#9013)

* Rename PostCSS files to `.pcss`

* Make Stylelint happy

* Delint

* Rename new files too

* delint

* Fix bad comment placement

res/themes/light/css/_paths.pcss

@@ -1,3 +1,3 @@
-// Path from root SCSS file (such as `light.scss`) to `res` dir in the source tree
+// Path from root SCSS file (such as `light.pcss`) to `res` dir in the source tree
 // This value is overridden by external themes in `element-web`.
 $res: ../../..;

res/themes/light/css/light.pcss

@@ -0,0 +1,7 @@
+@import "../../../../res/css/_font-sizes.pcss";
+@import "_paths.pcss";
+@import "_fonts.pcss";
+@import "_light.pcss";
+@import "_mods.pcss";
+@import "../../../../res/css/_components.pcss";
+@import url("highlight.js/styles/atom-one-light.css");

res/themes/light/css/light.scss

@@ -1,7 +0,0 @@
-@import "../../../../res/css/_font-sizes.scss";
-@import "_paths.scss";
-@import "_fonts.scss";
-@import "_light.scss";
-@import "_mods.scss";
-@import "../../../../res/css/_components.scss";
-@import url("highlight.js/styles/atom-one-light.css");