feat(tags): add mounted

[sebkolind]

This PR adds a mounted callback to attrs on tags. This is useful for mounting external libraries to the element when it's ready.

As an example you could use it like this to use https://codemirror.net:

div("", {
  // `el` is the element that `mounted` is added to
  mounted({ el }) {
    new EditorView({
      parent: el,
      extensions: [basicSetup, javascript()],
      doc: `function greet() {}`,
    });
  },
});