feat: allow to define lefthook executable or command

[mrexox]

Closes #443

⚡ Summary

There are two scenarios when you want to specify the lefthook executable or lefthook command:

1. You want to make sure developers in your project use the same lefthook version as installed in your project dependencies (makes sense when installing lefthook via npm, Ruby gems, PyPi)
2. You use PnP loader and have your package.json with lefthook dependency in a subdir. Currently there's no way to correctly do cd subdir; yarn run lefthook. This option can help with this.

The other special case is: you want to use your own lefthook executable defined in lefthook-local.yml (for whatever reason).

• Add lefthook field to the top-level config options
• Update hook template. Strip and replace newlines with ';' since the value will be wrapped with sh -c
• Add tests
• Add docs

So, examples:

Some hard-coded binary

# lefthook.yml

lefthook: /usr/bin/lefthook

pre-commit:
  jobs:
    - run: echo {staged_files}

Some hidden package

# lefthook.yml

lefthook: |
  cd subdir
  yarn run lefthook

pre-commit:
  jobs:
    - run: echo {staged_files}

Force using lefthook gem

# lefthook.yml

lefthook: bundle exec lefthook

pre-commit:
  jobs:
    - run: echo {staged_files}

[fmal]

thanks!