add vite.config.js, remove legacy code, improve readme.md

vite.config.js

@@ -0,0 +1,11 @@
+import { defineConfig } from 'vite';
+import { resolve } from 'path';
+ export default defineConfig({
+  build: {
+    lib: {
+      entry: resolve(__dirname, './src/wc-wysiwyg.ts'),
+      name: 'WcWysiwyg',
+      fileName: (format) => `wc-wysiwyg.${format}.js`,
+    },
+  },
+});