.gitignore Laravel Mix artifacts

[apolopena]

Describe the bug

Laravel Mix generates some files (for development) that should not be pushed to the repo as they will cause conflicts between developers.
Add these to the .gitignore

/public/css
/public/build
/public/mix-manifest.json

Screenshots

Steps to reproduce

Push the generated project scaffoding to your repo
Run yarn run dev
public/js/app.js will have changes in it.

Expected behavior

All Laravel Mix artifact files should be in .gitignore

Additional information

[apolopena]

Will be addressed in #42