feat: add MarkdownViewer component and cnYFM helper

[d3m1d0v]

Summary by Sourcery

Add a new markdown viewer package that renders pre-transformed HTML with YFM styling support and integrate it into the monorepo tooling and demo.

New Features:

• Introduce the @gravity-ui/markdown-viewer package exposing a MarkdownViewer React component for rendering HTML content and a cnYFM helper for YFM-specific classnames.
• Add Storybook demo stories showcasing markdown viewing via the new MarkdownViewer component.

Enhancements:

• Wire the new viewer package into the shared classname utilities and BEM naming scheme used across the project.

Build:

• Register the new viewer package in release-please configuration and publish workflow, and add its build configuration, gulpfile, and TypeScript setup.

[sourcery-ai]

Reviewer's Guide

Introduces a new @gravity-ui/markdown-viewer package that provides a MarkdownViewer React component for rendering pre-transformed HTML with YFM-aware styling helper cnYFM, wires it into the monorepo release/publish/demo setup, and adds basic build configuration and Storybook demo.

File-Level Changes

Change	Details	Files
Add @gravity-ui/markdown-viewer package with MarkdownViewer component for rendering HTML content and expose YFM classname helper.	Create new package metadata, scripts, build outputs, and dependencies for @gravity-ui/markdown-viewer. Implement MarkdownViewer React component that wraps dangerous inner HTML rendering inside a styled container and forwards ref/style/className. Introduce BEM classname utilities for viewer-specific classes and YFM-scoped styling via cnYFM helper, including YFMMods type. Set up TypeScript config, gulp build pipeline, and publication sideEffects for CSS/SCSS in the new package. Export MarkdownViewer, MarkdownViewerProps, cnYFM, and YFMMods from the package entrypoint.	packages/viewer/package.json packages/viewer/src/components/MarkdownViewer/MarkdownViewer.tsx packages/viewer/src/components/MarkdownViewer/index.ts packages/viewer/src/components/index.ts packages/viewer/src/utils/classname.ts packages/viewer/src/utils/cn-yfm.ts packages/viewer/src/index.ts packages/viewer/gulpfile.mjs packages/viewer/tsconfig.json packages/viewer/.prettierignore
Integrate markdown viewer into repo tooling, release workflow, and demo Storybook.	Register new viewer package in release-please manifest and config and hook it into the GitHub publish workflow matrix. Add @gravity-ui/markdown-viewer as a workspace dependency for the demo app. Create a Storybook Viewer component that runs @diplodoc/transform on markdown to HTML and renders it via MarkdownViewer with YFM styling. Add a Storybook story for the Markdown Viewer using existing demo markdown content.	.release-please/manifest.json .release-please/config.json .github/workflows/publish.yml demo/package.json demo/src/stories/viewer/MarkdownViewer.tsx demo/src/stories/viewer/MarkdownViewer.stories.tsx pnpm-lock.yaml packages/viewer/README.md

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[gravity-ui]

[gravity-ui]