Remove reliance on ms-vscode-remote.remote-ssh in vscode-coder

[f0ssel]

Related to: coder/vscode-coder#408

We need to create a Coder extension variant that is compatible with VSCodium, Cursor IDE, Windsurf, and other VS Code forks by implementing our own remote-ssh client in our plugin code. Because IDEs like Windsurf use neither Microsoft or Open Remote SSH extensions we ideally should not rely on existing IDE extensions but just the code in our plugin.

[bcpeinhardt]

I'm estimating this at a IV because anything less feels like a promise we may not be able to keep. I'm guessing this is actually a III, but I just don't have enough detail yet.