Publish the VSCode extension to Open VSX Registry

[oskar-gmerek]

Problem

Currently, users of open-source builds of VS Code (such as VSCodium, etc) cannot easily install this extension because these editors use the Open VSX Registry instead of the proprietary Microsoft Visual Studio Marketplace.

Solution

Publishing MasterCSS VSCode extension to open-vsx.org registry.
Since Open VSX is fully compatible with VS Code extensions, no code changes in the extension itself are required.