Don't commit auto-generated code

[letmaik]

src/polyfills.ts and polyfills/constants.js are generated during npm run build so there shouldn't be a need to commit them to the repo.

[rpadaki]

This also deals with the fact that constants.js has different values depending on your OS!

I'm currently trying to write a PR on Linux, and I had to manually build everything and skip the build:constants step to avoid completely rewriting constants.js and generating an unnecessary diff.

[FredKSchott]

I would love a PR to ignore