feat: add experimental unflatten handler for image processing

[nathanchase]

This pull request introduces a new handler function to the src/handlers/handlers.ts file. The new function, unflatten, is experimental and its API may change in the future.

New handler function added:

• src/handlers/handlers.ts: Added the unflatten handler, which calls the unflatten method on the provided pipe. This feature is marked as experimental.

See https://sharp.pixelplumbing.com/api-operation#unflatten for more information.

[pi0]

Thanks for PR dear @nathanchase. Wondering is it worth to wait for sharp feature to be stable first?

Do you have some more context why it is in experimental state?

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 56.18%. Comparing base (f666cb4) to head (7dd635f).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
src/handlers/handlers.ts	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #248      +/-   ##
==========================================
+ Coverage   56.02%   56.18%   +0.16%     
==========================================
  Files          14       14              
  Lines        1203     1212       +9     
  Branches       83       84       +1     
==========================================
+ Hits          674      681       +7     
- Misses        526      528       +2     
  Partials        3        3              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles

[nathanchase]

Thanks for PR dear @nathanchase. Wondering is it worth to wait for sharp feature to be stable first?

Do you have some more context why it is in experimental state?

I don't have any knowledge of why it's experimental. I just know that I have an immediate use case for it with https://github.com/nuxt/image to make all images with a solid white background become transparent backgrounds. I happened to notice it wasn't wired up to sharp's implementation, so this was just meant as a means of connecting to the unflatten functionality that already exists so that I could use it.

[nathanchase]

For what it's worth, unflatten has been in sharp since April 2023.
lovell/sharp#1648 (comment)