Switch to .mjs for ES Modules. Fixes #88.

src/lib/fix-pmc.mjs

@@ -0,0 +1,26 @@
+import { options } from 'preact';
+
+const classNameDescriptor = {
+  enumerable: false,
+  configurable: true,
+  get () {
+    return this.class;
+  },
+  set (value) {
+    this.class = value;
+  }
+};
+
+const old = options.vnode;
+options.vnode = vnode => {
+  const a = vnode.attributes;
+  if (a != null) {
+    if ('className' in a) {
+      a.class = a.className;
+    }
+    if ('class' in a) {
+      Object.defineProperty(a, 'className', classNameDescriptor);
+    }
+  }
+  if (old != null) old(vnode);
+};