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.

Refactors shared logic (text processing, attr building, element
node processing) into toRedactorHelpers.ts so both sync and async
versions are thin recursive shells with no duplicated code.

The existing sync jsonToHtml behavior is unchanged.

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 }