Revert CustomPluginOptions to be an interface

[sapphi-red]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

#5591 changed CustomPluginOptions type to be an Record, but that made it impossible to extend this type by declaration merging.
This PR reverts that change.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
rollup	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 24, 2025 6:00am

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (b4cb7cc) to head (0523222).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5850   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files         269      269           
  Lines        8545     8545           
  Branches     1467     1467           
=======================================
  Hits         8421     8421           
  Misses         92       92           
  Partials       32       32           

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[lukastaegert]

Interesting that this makes a difference, but sure.

[github-actions]

This PR has been released as part of rollup@4.34.9. You can test it via npm install rollup.