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.