feat: export CSS as string module for Shadow DOM injection

[makhnatkin]

Summary

Adds `@gravity-ui/markdown-editor/styles-string` — a JS module that exports all editor styles as a string, enabling injection into Shadow DOM via `adoptedStyleSheets` or a `<style>` tag.

```ts
import editorStyles from '@gravity-ui/markdown-editor/styles-string';

// Option 1: Constructable Stylesheets (modern browsers)
const sheet = new CSSStyleSheet();
sheet.replaceSync(editorStyles);
shadowRoot.adoptedStyleSheets = [sheet];

// Option 2: <style> tag (broader compatibility)
const style = document.createElement('style');
style.textContent = editorStyles;
shadowRoot.appendChild(style);
```

What's included

The exported string contains:

• All editor SCSS compiled from `src/**/*.scss`
• External CSS from `@diplodoc/*` packages imported directly in TS source (YfmCut, YfmFile, YfmTabs, YfmConfigs, QuoteLink, FoldingHeading)

Note: `@gravity-ui/uikit` styles are not included — if your shadow root also uses UIKit components, you'll need to inject those separately.

Implementation details

• Build: new `styles-string` gulp task runs after `scss`, reads `build/styles.css` + resolves external CSS via `require.resolve()`, writes `build/styles-string.mjs` (ESM) and `build/styles-string.cjs` (CJS)
• `.mjs` extension is used for the ESM file so Node.js always treats it as ESM, regardless of the `build/` root having no `"type": "module"`
• Smoke test added to `tests/esbuild-test/esbuild-tester.js` verifying both CJS and ESM files before the bundler compatibility test

Test plan

• `pnpm run build` — `build/styles-string.mjs`, `.cjs`, `.d.ts` are generated
• CJS: `typeof s === 'string' && s.length === 122371`
• `pnpm run test:esbuild` → `styles-string smoke test: OK (length: 122371)`

---

Closes #1026

Hey @zeeeeby — is this what you had in mind? The exported string covers core editor styles plus the default YFM preset's external dependencies. Let me know if anything is missing from your specific setup (e.g. which extensions/preset you're using, whether you also need UIKit styles injected).

🤖 Generated with Claude Code

[gravity-ui]

[gravity-ui]