The `LexicalMarkdownShortcutPlugin` package extends the defaults from `@lexical/markdown` with a HR transform but didn't expose that default list making it harder for anyone wanting to use the defaults plus their own extensions.
* Update LexicalMarkdownShortcutPlugin.d.ts
In https://github.com/facebook/lexical/pull/2124 the `transformers` prop was made optional, but the type was never made optional. This fixes that!
* Update LexicalMarkdownShortcutPlugin.js.flow
* Update LexicalMarkdownShortcutPlugin.jsx