feat: add jsonToHtmlAsync for async customElementTypes support

Add toRedactorAsync (exported as jsonToHtmlAsync) that supports
customElementTypes handlers returning string | Promise<string>.
Enables dynamic component resolution (e.g. await import()) before
serialization. Children are resolved via Promise.all concurrently.

The existing sync jsonToHtml is unchanged — fully backward compatible.

New types: IJsonToHtmlAsyncElementTags, IJsonToHtmlAsyncOptions.

Author: glassdimlygr

src/index.tsx

@@ -1,6 +1,7 @@
 import "array-flat-polyfill"
 import { fromRedactor } from "./fromRedactor"
 import { toRedactor } from "./toRedactor"
+import { toRedactorAsync } from "./toRedactorAsync"
 import {jsonToMarkdownSerializer} from './jsonToMarkdown'
 export * from "./types"
-export { fromRedactor as htmlToJson, toRedactor as jsonToHtml, jsonToMarkdownSerializer as jsonToMarkdown }
+export { fromRedactor as htmlToJson, toRedactor as jsonToHtml, toRedactorAsync as jsonToHtmlAsync, jsonToMarkdownSerializer as jsonToMarkdown }