Add gitattributes file

[chriswilty]

Currently, when cloning the repo the code is checked out using whatever line-ending is configured in each of our git configs, but prettier is expecting line-ending to be LF.

This means a prettier run can cause a load of errors, and touch a load of files in cleanup, which then disappear after git add, which is likely to be confusing. Aside from that, we don't want a line-endings war, so it makes sense to set our preference in the repo.

If we add a .gitattributes to the repo with this info, then all checkouts will respect that. Problem solved:

* text eol=lf

[chriswilty]

Small problem with font files to address...