Support for pdf links with named destinations

[przemech]

This adds support for named destinations described here:

mozilla/pdf.js#5258 (comment)

This also solves following issues:

• Open specific pdf page via markdown link #129
• open PDFs on a specific page by Markdown-Link #108

[przemech]

This will work only with absolute filepath links:
file:///<absolute_path>/<filename>.pdf#<named_destination>

When using a relative path in the vscode, the path is resolved and the #<named_destination> is trimmed, so it gets lost before it reaches extension's code. Anyway, such link will not work on a GitHub repo, so proposed change is just a minor feature.

[JBoysen1]

Yes - we need this. I use similar command line calls to open pdf in browsers. What would the VS code syntax on the command line look like ?