fix: deduplicate `formats`

[Leon338]

Hey!

I noticed a small bug where webp sources were showing up twice inside NuxtPicture if you explicitly added them to your config (it was merging with the defaults and doubling up).

Quick question: I also thought about just removing the webp default entirely if the user provides a custom config, but I wasn't sure if that would be considered a breaking change. Let me know if you think that's a cleaner path!

[pkg-pr-new[bot]]

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npm i https://pkg.pr.new/@nuxt/image@2038

commit: b9790e5

[codecov-commenter]

Codecov Report

:x: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 6.99%. Comparing base (db87207) to head (b9790e5).

Files with missing lines	Patch %	Lines
src/module.ts	0.00%	3 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #2038      +/-   ##
========================================
- Coverage   6.99%   6.99%   -0.01%     
========================================
  Files         78      78              
  Lines       3629    3632       +3     
  Branches     140     140              
========================================
  Hits         254     254              
- Misses      3326    3329       +3     
  Partials      49      49              

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