Add warning not to add assets directly to the bundle

[lukastaegert]

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

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[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#warning-for-this-emitfile

or load it into the REPL:
https://deploy-preview-4831--rollupjs.netlify.app/repl/?pr=4831

[codecov]

Codecov Report

Merging #4831 (18bf9a5) into master (669f597) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4831   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files         219      219           
  Lines        7839     7839           
  Branches     2171     2171           
=======================================
  Hits         7760     7760           
  Misses         26       26           
  Partials       53       53           

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[rollup-bot]

This PR has been released as part of rollup@3.12.1. You can test it via npm install rollup.