feat: open save as dialogue on export

[voxpelli]

This pull request improves the user experience when exporting a Markdown preview to HTML in the VS Code extension. Instead of automatically saving the HTML file to the system's temporary folder, users are now prompted to choose a save location and filename, with a sensible default based on the original document.

This helps when eg. exporting multiple markdown previews after one another, as previously they overwrote each other.

Enhancements to Markdown export workflow:

• Users are now prompted with a save dialog to choose the export location and filename for the HTML file, using the original document's name as a default suggestion.
• The confirmation message after export now displays the exported filename instead of the full file path, making it clearer and more user-friendly.