Modernize ep_highlight_excerpt to deal with block content

[felipeelia]

Description of the Change

Closes #3695

How to test the Change

Changelog Entry

Fixed - Infinite loop when using excerpt highlighting with posts that use blocks that print an excerpt

Credits

Props @felipeelia @JakePT

Checklist:

• [ ] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.