[WIP] Major 5 manifest

## Improve plugins functionality
- [ ] Implement async transform API (3y3)
- [ ] Add deprecation warning for sync transform API (3y3)

## Reduce package build size
- [x] Extract liquid to @diplodoc/md-liquid package (3y3)
- [ ] Add deprecation warning for liquid usage from current package (3y3)
- [x] Allow to replace liquid implementation in transformer (3y3)
- [x] Extract lint to @diplodoc/md-lint package (3y3)
- [ ] Add deprecation warning for lint usage from current package (3y3)
- [ ] Reimplement highlighte.js connection (?)

## Syntax
- [ ] ? Add `sup` by default
- [ ] ? Add `underline` by default
- [ ] ? Add `task list` by default
- [ ] Make markdown-attrs disabled by default. Add enable flag.

## API
- [ ] ? Migrate to ESM with cjs compatibility
- [ ] Limit exported API by package.exports