refactor(shell-proxy)!: rename env vars to `SHELLPROXY_*` and add usage message (#10456)

BREAKING CHANGE: the `DEFAULT_PROXY` setting has been renamed to `SHELLPROXY_URL`,
and `CONFIG_PROXY` has been renamed to `SHELLPROXY_CONFIG`. See the plugin README
for more information.

Co-authored-by: Marc Cornellà <hello@mcornella.com>