Add git attributes to force only lf line endings

[prestoncarman]

In setting up GitHub Actions, I noticed that bundle exec rubocop -D . was throwing 25 offense for something like this:

Gemfile:1:1: C: Layout/EndOfLine: Carriage return character detected.
source "https://rubygems.org" ...
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I added a .gitattributes file to force lf line endings to match the rubocop settings in .rubocop.yml

[prestoncarman]

The issue will show up once #200 has been included. Check out the log from https://github.com/prestoncarman/arduino_ci/runs/1404340103?check_suite_focus=true. I guess this has to do with either Windows version or configuration settings for Windows CI test.

[ianfixes]

Thanks!