Incompatibilities with VIP's dev-env

### Description of the bug

There are a few things about how this repo is set up that makes it not work properly with VIP's dev-env out of the box:

1. `.wpvip/vip-dev-env.yml` isn't gitignored, and this file will end up being unique to each user (due to user-specific paths)
2. `mu-plugins/001-composer.php` attempts to read `composer.json`, which isn't readable on VIP's dev-env

We should add `.wpvip/vip-dev-env.yml` to the .gitignore and we should add a guard in 001-composer.php so that it doesn't attempt to read a file that it can't access.

### Steps To Reproduce

1. Create a repo using this template repo as a basis.
2. Run the setup steps and choose WordPress VIP as the target.
3. Create a local dev environment using VIP's dev-env.
4. Use the repo you created as the wp-content folder.
5. Run `composer install` to install dependencies.
6. Attempt to load the site in a browser and notice that there is an error message about not being able to access composer.json.

### Additional Information

_No response_

Start by checking .gitignore and mu-plugins/001-composer.php, then reproduce the setup with WordPress VIP's dev-env and the repository as the wp-content folder. Done means .wpvip/vip-dev-env.yml is ignored and 001-composer.php no longer attempts to read an inaccessible composer.json; rerun the site load to confirm the error is gone.

Rédigé par le modèle d'indexation à partir du texte de l'issue.