Omit tests from npm bundle

This is a fairly large module on NPM at 9.6MB installed (see npmx). It looks like there's some low-hanging fruit for improving its bundle size: one major thing is that the NPM module includes test files. Excluding test files from the NPM build shrinks it by about 10% in my testing.

The simplest way to do this would be to add *.test.ts to the npmignore file to keep those files out of the built package.

It's less clear to me, but it also looks like the build/ directory could be excluded to save another 2.3MB, and the build-scripts directory to save another 66kB.

Happy to roll PRs for these if helpful.