Publish vscode-coder extension on Open VSX

[f0ssel]

Related to: coder/vscode-coder#408

Relies on: #430

Once we no longer rely on the MS extension, we should add publishing to OpenVSX to our build pipeline so we are easily available on other vscode-like IDEs.

[bcpeinhardt]

Publishing to the official VSCode Marketplace is manual for us right now, we physically upload a .vsix file. We should prioritize automating releases, but for the moment we'll likely just add a note to the contributing docs for how to publish to OpenVSX. I think we should make a companion ticket to this one for automating publishing to the official marketplace and tackle the two together.

[matifali]

duplicate of coder/vscode-coder#269