RFC: move YFM styles to separate package

[d3m1d0v]

Description

At the moment, styles for HTML content derived from YFM markup are in the package @diplodoc/transform. These styles include the basic markdown block styles and additional YFM block styles that are plugged together.

Problem: I cannot use pure markdown-it (or another markdown parser) with basic YFM styles without adding '@diplodoc/transform` dependency.

Proposal

Create a new package for styles – @diplodoc/styles (or @diplodoc/yfm-styles, @diplodoc/base-styles, etc), and move the base styles into it.

New package should include all styles for the base Markdown blocks, such as headings, quotes adn tables. It should also include HLJS styles add declaration of CSS-API as public CSS-variables/tokens; base styles for printing mode, color values for light and dark themes (in the feature).

This will allow to use styling of content from YFM without having to use the Diplodoc infrastructure.