[Feat]: make VSCODE_PROXY_URI use the subdomain proxy when it is enabled

[jsjoeio]

What is your suggestion?

When VSCODE_PROXY_URI is enabled, use the subdomain proxy.

Why do you want this feature?

Popular extensions like Tabnine can't use relative paths and need to be able to talk to code-server on specific ports/paths in order to work correctly.

Are there any workarounds to get this functionality today?

Port forwarding but this isn't always possible.

Are you interested in submitting a PR for this?

Yes, with more context.

[code-asher]

We might also want a way to override this for cases like Coder where we already provide a subdomain proxy outside of code-server. For this we can probably just check if that variable is already set and if so avoid overriding.

To implement we need to check the proxy-domain flag and use that in the environment variable. It can be defined multiple times so maybe we just use the first one. So more or less I think it would be {{port}}.${args["proxy-domain"][0]}. If the flag is not set we just keep using the path-based proxy.

I also think we should go ahead and patch asExternalUri to use this same environment variable although we should use the other ticket for that (and a separate PR).