chore: migrate to Vite+ toolchain

[fadi-george]

Summary

• Build: Replace tsc + manual copy with vp pack (tsdown). Output is a single CJS bundle at dist/index.js (~24 KB) with generated .d.ts types. Removes app.plugin.js shim.
• Test: Migrate all tests from bun:test to Vitest (vite-plus/test). Co-locate test files next to source modules instead of a separate __tests__/ directory.
• Lint/Format: Add vp check with type-aware linting (oxlint + tsgolint) and vp fmt (oxfmt). Fix all lint errors across src/ and examples/.
• Dependencies: Remove @types/bun, prettier, expo-module-scripts, concurrently, typescript from devDeps. Add @types/node, @types/react, vite-plus, @vitest/coverage-v8. Move @expo/config-plugins to peerDependencies.
• Project structure: Move serviceExtensionFiles/ from src/support/ to package root (static assets don't belong in the build pipeline). Add root tsconfig.json. Export OneSignalPluginProps type from package entry.
• Demo app: Add custom index.js entry point to fix Metro resolution under bun's nested node_modules layout.

Test plan

• vp test — 49 tests pass
• vp check src examples — no errors or warnings
• vp pack — builds dist/index.js + dist/index.d.ts
• bun run ios from examples/demo — verify app loads and plugin applies
• bun run android from examples/demo — verify app loads and plugin applies
• bun run ios from examples/disable-nse — verify NSE is not added

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