Convert the GitLab Vale styles directory `gitlab_base` into a Vale package

Convert the GitLab Vale styles directory gitlab_base into a Vale package

The GitLab Vale styles directory gitlab_base contains rules that are intended to be used across all GitLab documentation, regardless of the project. Vale has a feature called a package:

Packages provide a means of sharing, extending, syncing, and updating Vale configurations.

Converting the GitLab Vale styles directory gitlab_base into a package would allow it to be shared more easily, both inside and outside GitLab.

This came up in discussion in gitlab-org/security-products/secret-detection/secret-detection-rules!39 (merged) where Vale is used to check Markdown content.