Update build scripts for new process

We have to convert *something* to TypeScript so it doesn't complain that there's nothing to compile, so this converts the easiest utility library.

Many of the scripts are copied from the react-sdk.

.stylelintrc.js

@@ -0,0 +1,26 @@
+// Copied from react-sdk
+// TODO: Only keep one copy of this for synchronization purposes
+module.exports = {
+    "extends": "stylelint-config-standard",
+    "plugins": [
+        "stylelint-scss",
+    ],
+    "rules": {
+        "indentation": 4,
+        "comment-empty-line-before": null,
+        "declaration-empty-line-before": null,
+        "length-zero-no-unit": null,
+        "rule-empty-line-before": null,
+        "color-hex-length": null,
+        "max-empty-lines": null,
+        "number-no-trailing-zeros": null,
+        "number-leading-zero": null,
+        "selector-list-comma-newline-after": null,
+        "at-rule-no-unknown": null,
+        "no-descending-specificity": null,
+        "scss/at-rule-no-unknown": [true, {
+            // https://github.com/vector-im/riot-web/issues/10544
+            "ignoreAtRules": ["define-mixin"],
+        }],
+    }
+};