Runtime helper library

[intrnl]

A runtime helper library like tslib and babel-runtime

This can help deduplication when used in other bundlers like Webpack or Rollup, especially for libraries bundled with esbuild where it has the same runtime helper dependency.

[evanw]

Closing as a duplicate of #1230. Even though this issue came first, that issue has more information and a longer discussion, so I'm going to keep that one open instead.