Ensure syntax-mcfunction is published on Open VSX

[SPGoding]

Upstream issue: MinecraftCommands/syntax-mcfunction#11

The extensionDependencies field in packages/vscode-extension/package.json would have to be excluded when releasing to Open VSX if syntax-mcfunction is not published there when v4 is ready for release.

Note that https://github.com/open-vsx/publish-extensions could be a temporary way to publish the extension if upstream is unresponsive.