ci: Add style check for 'david' with an i

Check the code for misspellings of 'dav1d'/'DAV1D' as 'david'/'DAVID'.
This helps consistency and prevents bugs arising from these
misspellings.

.gitlab-ci.yml

@@ -11,6 +11,7 @@ style-check:
         - amd64
     script:
         - git grep -n -P "\t|\r| $" -- . ':(exclude)*/compat/*' && exit 1
+        - git grep -n -i "david" -- . ':(exclude)THANKS.md' ':(exclude).gitlab-ci.yml' && exit 1
         - git remote rm upstream 2> /dev/null || true
         - git remote add upstream https://code.videolan.org/videolan/dav1d.git
         - git fetch -q upstream master