Add docs, code coverage and tests to .npmignore

@proverma Cleaning up the .npmignore for Arrow. The package that gets installed by users has a bunch of files that are test files/docs. Adding these to npmignore also brings down the size of the package by 4.6MB
```
coverage
demo
docs
index.js
lcov-report
lcov.info
nodejs
results.xml
tests
tmp
ytestrunner.json
```