feat: support postcss (#600)

Author: baseballyama

.changeset/silver-swans-build.md

@@ -0,0 +1,5 @@
+---
+"svelte-eslint-parser": patch
+---
+
+feat: support postcss

src/parser/style-context.ts

@@ -56,6 +56,7 @@ export function parseStyleContext(
   let parseFn: Parser<Root>, sourceAst: Root;
   switch (sourceLang) {
     case "css":
+    case "postcss":
       parseFn = postcss.parse;
       break;
     case "scss":

tests/fixtures/parser/ast/postcss-input.svelte

@@ -0,0 +1,6 @@
+<style lang="postcss">
+  body {
+    colour: white;
+    background-colour: grey;
+  }
+</style>