Support in-page anchor links (#fragment) for Markdown headings

[liyofx]

Currently, flutter_markdown / flutter_markdown_plus correctly parses Markdown links like:

[Developer Tools](#developer-tools)

However, when such links are tapped, the framework treats them as external URLs and attempts to launch them via platform intents, which results in errors such as:

PlatformException(ACTIVITY_NOT_FOUND, No Activity found to handle intent { #developer-tools })

There is currently no built-in support for in-page anchor navigation, i.e. scrolling to a heading within the same Markdown document.

Expected Behavior

Links that start with # should be treated as internal anchor links

Tapping an anchor link should scroll to the corresponding heading in the same Markdown view

Headings (#, ##, ###, etc.) should be able to expose a stable anchor ID (similar to GitHub-style slug generation)

Example:

## Developer Tools
...
[Go to Developer Tools](#developer-tools)

Tapping the link should scroll to the ## Developer Tools section.