Modernize build tooling: esbuild + pnpm and add multi-version CI testing

[EhabY]

Summary

This PR modernizes the build infrastructure, resulting in >2x faster CI builds through better caching (pnpm) and faster tooling (esbuild).

Build System

• Replace Webpack with esbuild for extension bundling - (x5 - x10 improvement)
• Add esbuild.mjs configuration with watch mode support and openpgp CJS alias fix
• Remove webpack, webpack-cli, and ts-loader dependencies
• Add --no-dependencies to vsce packaging (dependencies are bundled by esbuild)

Package Manager Migration

• Migrate from yarn to pnpm - enables better dependency caching in CI
• Add pnpm-workspace.yaml with onlyBuiltDependencies for native modules
• Enable caching for prettier and eslint (--cache --cache-strategy content)

CI/CD Improvements

• Use pnpm/action-setup@v4 with built-in caching
• Remove global npm install -g @vscode/vsce - use local devDependency instead
• Add multi-version test matrix:
  • Unit tests: Electron 32 (VS Code 1.95 minimum) and latest
  • Integration tests: VS Code 1.95.0 and stable
• Add scripts/test-electron.sh for version-specific test runs

Other Changes

• Replace @peculiar/x509 with node-forge for certificate parsing
• Update minimum VS Code engine: 1.73.0 → 1.95.0
• Update @types/node to v20, add explicit node >= 20 engine requirement