Add gitattributes to reduce unnecessary files from composer installs.

[jdufresne]

When using the composer argument --prefer-dist, git archives will be used. This reduces files that are typically useless when using Twig as a library, as opposed to developing it. When using Twig as a library, and deploying, tests and raw documentation are unnecessary. When developing Twig, the preferred method is a git clone, and will continue to work properly.

[fabpot]

see symfony/symfony#5674, it was the case some time ago and it was reverted.

[fanno]

I would like to see this change. =/ I don't see why those who wants the docs could not just use clone or composer.....--prefer-source

Not adding this means i manually have to go throw vendors deleting unneeded "crap" when building for deployment... not every host out there allow for uploading packages of huge size for installs. in some case 2 MB is max size and is really do from time to time.

so i agree with:
symfony/symfony#6605 (comment)
and
symfony/symfony#6605 (comment)

[fabpot]

Closing as this PR is on the wrong repo. Someone else opened a PR on the official repo (twigphp#1776)