feat: add Tailwind CSS and PostCSS configuration

- Created postcss.config.js to configure PostCSS with Tailwind CSS and Autoprefixer.
- Updated svelte.config.js to enable PostCSS preprocessing.
- Added tailwind.config.js for Tailwind CSS configuration.
- Updated yarn.lock to include new dependencies for Tailwind CSS, PostCSS, and related plugins.

postcss.config.js

@@ -0,0 +1,6 @@
+module.exports = {
+    plugins: {
+        tailwindcss: {},
+        autoprefixer: {},
+    },
+}