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](https://github.com/csstools/postcss-language/blob/5d003170c5ed962b09b9a0f3725a6cae885df292/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",

crates/languages/src/css/config.toml

@@ -1,6 +1,6 @@
 name = "CSS"
 grammar = "css"
-path_suffixes = ["css", "postcss"]
+path_suffixes = ["css", "postcss", "pcss"]
 autoclose_before = ";:.,=}])>"
 brackets = [
     { start = "{", end = "}", close = true, newline = true },