[r5n-88] Modernize extension: React 17-19 support, performance fixes, updated snippets

[deralaxo]

Summary

Performance

• Removed bundled prettier dependency (was experimental, had race conditions, caused activation crashes). Snippet search now delegates formatting to the user's configured VS Code formatter
• Converted all synchronous file I/O to async
• Replaced JSON.stringify emptiness check with Object.keys in activation
• Removed arbitrary 1s setTimeout in config change handler
• Zero runtime dependencies

Snippet updates

• Added React 18 hooks: useId, useTransition, useDeferredValue
• Added React 19 hooks: useActionState, useFormStatus, useOptimistic, use
• Added 'use client' and 'use server' directive snippets
• Updated React Router imports for v6 (Switch removed, added Routes, useNavigate, useParams, useSearchParams, createBrowserRouter, useLoaderData, useFetcher)
• Added router setup snippets: rtrsetup (createBrowserRouter boilerplate), rtrla (route with loader/action)
• Added Redux Toolkit snippets: createAsyncThunk, RTK Query createApi, createSlice with extraReducers
• Changed default wrapper from <div> to <></> (Fragments) in all component snippets
• Changed importReactOnTop default to false (React 17+ JSX transform)
• Removed deprecated rfcreduxp snippet and broken imbrs (imported removed Switch)

Bug fixes

• Fixed extensionConfig to use explicit config.get() instead of unsafe cast — settings were not being read correctly
• Fixed snippet regeneration mutating source objects (Object.assign(snippet, ...) → Object.assign({}, snippet, ...)), which prevented settings changes from taking effect after first generation
• importReactOnTop now applies to all snippets consistently (was only applied to a hardcoded subset)
• Added languageScopes validation with user warning for invalid values
• Fixed activationEvents casing (typescriptReact → typescriptreact)
• Added missing export to tsrafc snippet to match its JS counterpart rafc

Documentation

• Rewrote README: removed dead badge links and sponsor section, updated options table, fixed repo URLs
• Rewrote Snippets.md: all 194 snippets now documented, removed ghost entries (imrs, imrse, imbrr, fref, ptypes), updated all examples to match current output

Issues

Fixes #348, fixes #347, fixes #346, fixes #344, fixes #320 — Performance / slow IDE
Fixes #333, fixes #325, fixes #313, fixes #305 — Extension crashes on startup
Fixes #350 — React import unnecessary with JSX transform
Fixes #316 — Snippets should use Fragments instead of <div>
Fixes #319 — Unknown language in activationEvents
Fixes #299 — Docs linking to deprecated reactjs.org

PR contributions incorporated

Closes #280 — Remove unnecessary React import (approach reworked)
Closes #291 — Update react-router imports to v6 (approach reworked)
Closes #317 — Redux API slice snippet (implemented as rxapi)
Closes #300 — Fix docs linking to old reactjs.org
Closes #301 — Missing export on tsrafc

[luislealdev]

Great work @deralaxo, #280 was something that really used to bother me if you ask and happy to be part of this great contribution.