[ty] Support generate-shell-completion (#17879)

## Summary

Resolves #15502.

`ty generate-shell-completion` now works in a similar manner to `ruff
generate-shell-completion`.

## Test Plan

Manually:

<details>

```shell
$ cargo run --package ty generate-shell-completion nushell
module completions {

  # An extremely fast Python type checker.
  export extern ty [
    --help(-h)                # Print help
    --version(-V)             # Print version
  ]
  
  # ...

}

export use completions *
```
</details>

crates/ty/src/args.rs

@@ -24,6 +24,10 @@ pub(crate) enum Command {
 
     /// Display ty's version
     Version,
+
+    /// Generate shell completion
+    #[clap(hide = true)]
+    GenerateShellCompletion { shell: clap_complete_command::Shell },
 }
 
 #[derive(Debug, Parser)]

crates/ty/src/main.rs

@@ -7,7 +7,7 @@ use std::sync::Mutex;
 use crate::args::{Args, CheckCommand, Command, TerminalColor};
 use crate::logging::setup_tracing;
 use anyhow::{anyhow, Context};
-use clap::Parser;
+use clap::{CommandFactory, Parser};
 use colored::Colorize;
 use crossbeam::channel as crossbeam_channel;
 use rayon::ThreadPoolBuilder;
@@ -68,6 +68,10 @@ fn run() -> anyhow::Result<ExitStatus> {
         Command::Server => run_server().map(|()| ExitStatus::Success),
         Command::Check(check_args) => run_check(check_args),
         Command::Version => version().map(|()| ExitStatus::Success),
+        Command::GenerateShellCompletion { shell } => {
+            shell.generate(&mut Args::command(), &mut stdout());
+            Ok(ExitStatus::Success)
+        }
     }
 }