Add a 'clean-exclude' option (to avoid cleaning node_modules)

Currently, the checkout action will delete node_modules with git clean -ffdx. Since I use (mostly) persistent self-hosted runners, it's extremely inefficient to constantly be deleting and recreating node_modules. It takes about a minute just to do the deletion in git clean and another 3 minutes to repopulate node_modules after that.

It would be much much faster to be able to pass the exclude flag to clean, for example:

git clean -ffdxe '/node_modules/'

# Or, more explicit...
git clean -ffdx --exclude='/node_modules/'

This could just be another argument to the action, so usage might be...

- name: Checkout
   uses: actions/checkout@v3
   with:
     clean-exclude: /node_modules/

Start by tracing the checkout action's existing clean option and the code that invokes git clean. Add support for the proposed clean-exclude input, then verify that its value is passed to git clean so paths such as /node_modules/ are preserved while other untracked files are cleaned.