fix(webClientServer): use relative path in updateUrl

[jsjoeio]

Description

This patches the webClientServer to ensure the updateUrl is relative to the root.
This ensures the correct URL in case code-server is being served behind a
reverse proxy.

An example of this would be using Caddy to server code-server on
localhost:8082/code/

This fix ensures request urls are created against localhost:8082/code/ instead
of localhost:8082/

Screenshot

Notice the Request URL in the image. Serving code-server regularly (i.e. on 8080), the update/check is made against the root.

When serving code-server behind a reverse-proxy with Caddy, the update/check is made against the reverse-proxy root (i.e. /code/

Testing Plan

I tested against the root and the reverse-proxy.

1. cd vscode && git checkout jsjoeio-fix-csp-reverse-proxy
2. yarn link
3. cd code-server && yarn link code-oss-dev --modules-folder vendor/modules
4. create Caddyfile somewhere:

http://localhost:8082/code/* {
  uri strip_prefix /code
  reverse_proxy 127.0.0.1:8080
}

5. caddy run
6. Navigate to http://localhost:8082/code/ in the browser and check Network tab. Filter for "check" and look at the Headers tab for the request.

I also did some basic testing around new URL in Codesandbox here.

This PR fixes coder/code-server#4476

(There is a chance this fixes other reverse proxy issues, but I will test after this is merged).

[jsjoeio]

NOTE: transitioning to draft so I can come up with a more reliable solution.

Look at http in code-server and add a new method that does this instead. We should be able to achieve this change with relative urls.

[jsjoeio]

@TeffenEllis since Asher is out today and next week, do you mind reviewing? It's very minimal