ESLint needs to ignore /vendor folder

Migrated issue

Reported by: jonathan1055

Related to !382 (merged) !376 (merged)

Problem/Motivation

Gitlab Templates own "check code" job has just started failing on eslint, but the files are not in this project. All of the errors are in files under /builds/issue/gitlab_templates-3492951/vendor/

See https://git.drupalcode.org/issue/gitlab_templates-3492951/-/jobs/5658781#L190

Proposed resolution

We need to avoid checking files in the /vendor folder. This can be done with a .eslintignore config file.

Remaining tasks