Use inline postcss config in vite.config.mts

1. It's redundant to have both a postcss.config.js and a postcss key in vite.config.mts
2. If the css key is present in vite.config.mts, it will override the postcss.config.js file (https://vitejs.dev/config/shared-options.html#css-postcss)
3. Sine the css key was overriden in vite.config.mts, and it didn't have autoprefixer, CSS was not being autoprefixed

vite.config.mts

@@ -2,6 +2,7 @@ import { defineConfig } from 'vite'
 import RubyPlugin from 'vite-plugin-ruby'
 import StimulusHMR from 'vite-plugin-stimulus-hmr'
 import tailwindcss from "tailwindcss"
+import autoprefixer from "autoprefixer"
 
 export default defineConfig({
   plugins: [
@@ -10,7 +11,10 @@ export default defineConfig({
   ],
   css: {
     postcss: {
-      plugins: [tailwindcss()],
+      plugins: [
+        tailwindcss(),
+        autoprefixer(),
+      ],
     }
   },
   optimizeDeps: { exclude: ["fsevents"] },