Silence `prefer_named_exports` warning when there are no named exports

[chocolateboy]

Set rollup's output.exports option to "auto" to avoid warnings about default exports when there are no named exports. Closes #425.

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[codecov[bot]]

Codecov Report

Merging #426 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #426   +/-   ##
=======================================
  Coverage   75.21%   75.21%           
=======================================
  Files          11       11           
  Lines         456      456           
  Branches      170      170           
=======================================
  Hits          343      343           
  Misses        108      108           
  Partials        5        5           

Impacted Files	Coverage Δ
src/index.ts	73.68% <ø> (ø)

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