Rewrite video tags on publish

[mjbvz]

From microsoft/vsmarketplace#494
For microsoft/vscode#134514

When publishing an extension readme, we supported rewriting relative image paths to absolute paths that the marketplace and VS Code can resolve. We should do the same for videos

This test extension demonstrates the issue. The markdown in the readme is:

Local video:

<video src="test.mp4"></video>

Local Image: 

<img src="cat.gif">

Remote video:

<video src="https://github.com/mjbvz/test-vscode-ext/raw/main/test.mp4"></video>

Unlike the <img> tags, the first video's src does not get correctly rewritten to point to the extension's GitHub repo