Correct .npmignore

This is a followup to !22 (merged), which contained an error resulting in not all source code being included in the released package (apologies 🙇). As per previous merge requests, I included a version bump.

The use of * caused nested files to be ignored as well, /* instead only ignores everything in the root directory. As a result, now nested files in the build/ directory will be included in the released package going forward.

I'm not sure if source maps (.map files) should be included in the released package. If not, build/**/*.map can be added after the !build/ line to filter those out.