Upload to Open VSX Marketplace for VS Open Source build compatibility

Some people use open source builds of VS Code core. One common example for data science is Posit's Positron.
The official Marketplace isn't available to these builds.

> Positron has an Extensions view, just like VS Code. The primary difference is that when you browse extensions in Positron, the extensions don’t come from VS Code’s Marketplace. Instead, they come from [Open VSX](https://open-vsx.org/), an Eclipse open-source project and vendor-neutral alternative to the Visual Studio Marketplace. This is necessary for licensing reasons; Microsoft doesn’t permit access to the Marketplace outside of their Visual Studio Code (VS Code) builds.

Can we upload to Open VSX also to enable more users access to this extension.