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.

cli/src/commands/context.rs

@@ -5,11 +5,11 @@
 
 use crate::{log, state::LauncherPaths};
 
-use super::args::Cli;
+use super::args::CliCore;
 
 pub struct CommandContext {
 	pub log: log::Logger,
 	pub paths: LauncherPaths,
-	pub args: Cli,
+	pub args: CliCore,
 	pub http: reqwest::Client,
 }