Move webapp related build files into the webapp directory

modules/webapp/postcss.config.js

@@ -0,0 +1,31 @@
+//postcss.config.js
+const tailwindcss = require("tailwindcss");
+
+const devPlugins =
+      [
+          require('postcss-import'),
+          tailwindcss("./tailwind.config.js"),
+          require("autoprefixer")
+      ];
+
+const prodPlugins =
+      [
+          require('postcss-import'),
+          tailwindcss("./tailwind.config.js"),
+          require("autoprefixer"),
+          require("@fullhuman/postcss-purgecss")({
+              content: [
+                  "./src/main/elm/**/*.elm",
+                  "./src/main/styles/keep.txt",
+                  "../restserver/src/main/templates/*.html"
+              ],
+              defaultExtractor: content => content.match(/[A-Za-z0-9-_:/\.]+/g) || []
+          }),
+          require('cssnano')({
+              preset: 'default'
+          })
+      ]
+
+module.exports = (ctx) => ({
+    plugins: ctx.env === 'production' ? prodPlugins : devPlugins
+});