Fix conditional exports in Node 13.0 - 13.7

[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:
Resolves #3427

Description

Apparently, conditional "require" exports were named "default" in earlier Node versions. This should enable importing Rollup in those versions again.

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#fix-node-13.7-require

or load it into the REPL:
https://rollupjs.org/repl/?circleci=9803

[codecov]

Codecov Report

Merging #3428 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3428   +/-   ##
======================================
  Coverage      95%     95%           
======================================
  Files         171     171           
  Lines        5831    5831           
  Branches     1722    1722           
======================================
  Hits         5540    5540           
  Misses        157     157           
  Partials      134     134

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