Treat .pcss files as CSS (#21402)

This addresses
https://github.com/zed-industries/zed/pull/19416#discussion_r1865019293
and also follows the [associated PostCSS file extensions for VS
Code](5d003170c5/package.json (L37)).

Release Notes:

- `.pcss` files are now recognized as CSS

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

assets/icons/file_icons/file_types.json

@@ -127,6 +127,7 @@
     "ogg": "audio",
     "opus": "audio",
     "otf": "font",
+    "pcss": "css",
     "pdb": "storage",
     "pdf": "document",
     "php": "php",