Use marked.js official heading id implementation

[SukkaW]

https://github.com/markedjs/marked-gfm-heading-id

Marked.js has an official plugin for that so we should use it instead.

Also, marked.js provides a new way of modifying the renderer's behavior, we should adopt it as well.

cc @jiangtj @stevenjoezhang @hexojs/core

[stevenjoezhang]

As of marked@5.0.0: headerIds and headerPrefix parameters enabled by default, but are deprecated since version 5.0.0, and will be removed in the future. To clear this warning, install https://www.npmjs.com/package/marked-gfm-heading-id, or disable by setting {headerIds: false}.