Publish to OpenVSX Registry

[radeksimko]

This would make it much easier to use the extension in project such as

• gitpod.io
• theia-ide.org
• VS Codium
• code-server

We can probably reuse most of the logic from hashicorp/vscode-terraform#1064 with the only difference that we don't need to account for platform-specific builds, so we may not need to wait for eclipse/openvsx#450 unlike in case of the Terraform extension.