Init Axo design system

ts/axo/_internal/css.tsx

@@ -0,0 +1,27 @@
+// Copyright 2025 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export type Styles = string & { __Styles: never };
+
+export function css(
+  ...classNames: ReadonlyArray<Styles | string | boolean | null>
+): Styles {
+  const { length } = classNames;
+
+  let result = '';
+  let first = true;
+
+  for (let index = 0; index < length; index += 1) {
+    const className = classNames[index];
+    if (typeof className === 'string') {
+      if (first) {
+        first = false;
+      } else {
+        result += ' ';
+      }
+      result += className;
+    }
+  }
+
+  return result as Styles;
+}