Remove dependency on string@3.3.3 which has a security vulnerability (ReDos)

[lgodmer]

There is a security vulnerability in string@3.3.3 which causes it to be flagged by tools like snyk.
jprichardson/string.js#212

The vulnerable methods in string@3.3.3 are not used by this project, however it still causes warnings from automated tooling. The issue has been discussed in the string.js repo for over 6 months and there is no traction there on a fix.

Consider switching to slugify (https://www.npmjs.com/package/slugify) instead, which does not have any open security vulnerabilities.

[martinlissmyr]

Fixed by https://github.com/Oktavilla/markdown-it-table-of-contents/releases/tag/v0.4.0 ... Thanks!

[lgodmer]

Thank you!