fix/#781-anchor-link-does-not-pass-valid-link-requirement

[rizaardiyanto1412]

Description

Add a validation to make the anchor link valid

Benefits

Possible drawbacks

Applicable issues

Solved: https://github.com/publishpress/PublishPress-Checklists/issues/781

Checklist

• [x] I have created a specific branch for this pull request before committing, starting off the current HEAD of development branch.
• [x] I'm submitting to the development, feature/hotfix/release branch. (Do not submit to the master branch!)
• [x] This pull request relates to a specific problem (bug or improvement).
• [x] I have mentioned the issue number in the pull request description text.
• [x] All the issues mentioned in this pull request relate to the problem I'm trying to solve.
• [x] The code I'm sending follows the PSR-12 coding style.