Add unused files to .npmignore

There is a lot of files in npm repository, causing it being 10MB+ (mainly due to unused `dist` folder). This is really annoying for CI usage or similar cases where this additional size causes unnecessary slow-down.

You can inspect results of this change on my release: https://www.npmjs.org/package/less-minimal. It should work exactly the same as node version only requires files in `lib` and `bin`