cli: implement better self-updating

- Start separating a "standalone" CLI. This is a little awkward with clap-
  derive, but I got it working. Detection of whether the CLI _is_
  standalone is still todo.
- Remove the old ad-hoc update code for code-server, and use the update
  service instead.
- Fix some of the "permission denied" errors people got while updating
  before. We need to rename the old running binary, not just overwrite it.

.vscode/settings.json

@@ -91,6 +91,9 @@
 		"editor.defaultFormatter": "rust-lang.rust-analyzer",
 		"editor.formatOnSave": true,
 	},
+	"rust-analyzer.linkedProjects": [
+		"cli/Cargo.toml"
+	],
 	"typescript.tsc.autoDetect": "off",
 	"testing.autoRun.mode": "rerun",
 	"conventionalCommits.scopes": [