chore: migrate from npm to pnpm

[EugeniyKiyashko]

Migrate the repository from npm to pnpm package manager.

Changes

Package management

• Added packageManager: "pnpm@9.15.9" to root package.json
• Created pnpm-workspace.yaml for workspace configuration
• Replaced package-lock.json (×3) with single pnpm-lock.yaml
• Moved overrides from root and packages/devextreme-cli into root pnpm.overrides
• Removed overrides from packages/devextreme-cli/package.json (pnpm only supports root overrides)

Lerna

• Configured lerna.json with "npmClient": "pnpm" and "useWorkspaces": true
• Removed postinstall: "lerna bootstrap" (pnpm handles workspace linking)
• Replaced lerna run commands with pnpm -r run / pnpm --filter
• Kept lerna as devDependency for publish workflow (lerna version + lerna publish)

Scripts

• Root: npm run --prefix ./packages/devextreme-cli <script> → pnpm --filter devextreme-cli run <script>
• Root: lerna run --stream test → pnpm -r run test
• Sub-packages: npm run → pnpm run in script chains

CI/CD workflows

• lint.yml: Added pnpm/action-setup@v4, switched to pnpm install --frozen-lockfile
• applications.yml: Same + updated argument passing, pnpm setup for schematics-test job
• themebuilder.yml: Replaced manual actions/cache with built-in pnpm caching
• publish.yml: npx lerna → pnpm exec lerna, single pnpm install for workspace
• check-nextjs.yml: Removed cache: 'npm' (this workflow tests end-user npm flow, not repo build)

Documentation

• Updated README_DEVELOPERS.md: all npm run → pnpm run, lerna run → pnpm --filter

Not changed (product code)

• src/utility/package-manager.js — end-user package manager detection (npm/yarn)
• src/utility/ng-version.js — npm view for Angular version lookup
• src/utility/package-json-utils.js — generates scripts for end-user apps
• src/applications/*.js — postinstall scripts in generated apps
• packages/devextreme-schematics/src/utility/typescript-resolver.ts — npm install/root commands