Feature: allow pre-parsed HTML nodes to be passed to HtmlWidget

[CaptainDario]

Adds parseHtmlToNodes() (a public, pure-Dart function) and a matching parsedNodes parameter on HtmlWidget, so users can offload the HTML parsing to an isolate and pass the result directly to the widget.

Why

Rendering large HTML snippets causes main-thread jank. The package already offloads HTML-string-to-DOM parsing to a compute() isolate when the string exceeds 10k chars.
However, in some scenarios it is helpful to have your own isolate for HTML generation/parsing/processing/fetching (e.g, slow HTML generation, fetching from web resources, ...). In this case, it would be useful to also do the HTML -> NodeList conversion in that isolate instead of sending the string back to the main isolate, and that isolate in turn sends it back to a processing isolate of flutter_widget_from_html.

What changed

• _parseHtml is now public as parseHtmlToNodes(String html) → NodeList. It has no Flutter dependencies and is safe to call inside any isolate.
• HtmlWidget accepts an optional parsedNodes parameter. When provided, the HTML-string-to-DOM parsing step is skipped in both the sync and async build paths, and buildAsync defaults to false. In this case, the HTML-string parameter is completely ignored.
• NodeList is re-exported from the core library so consumers don't need a direct package:html dependency.
• No breaking changes, all tests pass.

Usage

// Inside your existing background isolate:
final nodes = parseHtmlToNodes(myHtml);

...

// Back on the main thread — HTML→DOM parsing is already done:
HtmlWidget("", parsedNodes: nodes)

[CaptainDario]

@daohoangson Any chance for this to be merged?