add .gitattributes for GitLab syntax highlighting

7 jobs for master in 16 minutes and 22 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #15087
fp
build-coq.8.6.0

00:07:04

passed #15086
fp
build-coq.8.6.1

00:04:32

passed #15085
fp
build-coq.8.7.0

00:08:05

passed #15084
fp
build-coq.8.7.1

00:03:29

passed #15083
fp-timing
build-coq.8.7.2

00:03:20

passed #15082
fp-timing
build-coq.8.8.0

00:03:29

passed #15081
fp
build-coq.dev

00:16:22