Migration to v4

v4 has the same external API as v3, but significantly changed internals. Plugin authors should update their packages.

For users

External API did not change.

If you used markdown-it with plugins - make sure to update those.
If you modified renderer - see dev info below.
If you did not use plugins and renderer modification - no changes needed.

For developers

Tokens and renderer

Tokens are now classes, and allow arbitrary attributes.
- new tokens are created with token = state.push(type, tag, nesting). See this commit to understand how to create tokens in new way. Also see changes in plugins from other repos in this org.
Renderer methods were unified. Number of custom renderer rules were significantly reduced. Custom renderer functions need update due to tokens format change.

Other changes

.validateUrl() -> moved to root class .validateLink()
added .normalizeLink() & .normalizeLinkText() to root class, and removed normalizeUrl() from utils.
removed replaceEntities() in utils.