feat: add interrupt (Ctrl-C) handling

[mrcljx]

Closes #542

⚡ Summary

Hitting Ctrl-C during commit would terminate lefthook immediately, and stashed changes would not be restored. This can be improved by using a NotifyContext which prevents lefthook from being killed while still terminating subprocesses.

Test Plan

1. In this repo…
2. Added to .lefthook.toml

   [pre-commit.commands.sleep]
   run = "sleep 10 && exit 1"
   

3. Staged that change
4. Added # unstaged at the end of .lefthook.toml
5. Ran git commit -m 'test'
6. Hit Ctrl-C
7. Ran git status && git stash list
8. Before:
   • Output: ⠏ waiting: sleep^C
   • The unstaged comment was missing
   • Found stash@{0}: lefthook auto backup
9. After:
   • Output:

     ⠦ waiting: sleep^C
     
     EXECUTE > sleep
     
     Error: Interrupted
     

   • The unstaged comment was restored
   • Stash list was empty

☑️ Checklist

• Check locally
• Add tests
• Add documentation (N/A)

[mrcljx]

• Was able to reproduce the previous issue and it's fixed.
• The macOS integrity test should pass now (shouldn't have tested for SIGHUP)

[mrcljx]

Ah, well, celebrated too early. Pushed another fix (since bash is at /bin/bash for macOS):

- /usr/bin/bash
+ /usr/bin/env bash

[mrexox]

Thank you for doing this work! Going to release this change this week