Detect Word Internal Links and add title bookmark

[damienfa]

Description

At the moment, PhpWord is not able to read an internal link to an existing heading. This is because the anchor is not detected in the reader. Here I create a new (internal) "Link" when this attribute is found. Moreover, the Headings of the Word document usually contain a "bookmark" (useful for TOC or internal link) but the reader has not created it so that the name (reference of the bookmark) is lost.

Fixes # (issue)

https://github.com/PHPOffice/PHPWord/issues/2121

Checklist:

• [x] I have run composer run-script check --timeout=0 and no errors were reported
• [x] The new code is covered by unit tests (check build/coverage for coverage report)
• [ ] I have updated the documentation to describe the changes