Implement a check for lack of missing tags

Implement a check for lack of missing tags, independent of value labels (in comparison to `check_missing_range()` and `check_missing_regex()`). This could be a global check or a check that uses the missing range as specified in `check_all()` or looks for `NAs` in the data.

Start by reading check_missing_range(), check_missing_regex(), and check_all() to understand the existing missing-value checks and their entry points. Clarify whether the new check is global or uses the missing range, then define completion as detecting missing tags independently of value labels and covering the chosen behavior consistently.

Written by the indexing model from the issue text.