Refactoring for janus.js release on npm

[atoppi]

Since we have taken the ownership of the janus-gateway package on npm, this PR refactors the project for the release of janus.js in the npm public registry.

Potentially breaking changes

• package.json in the project root has been removed
• js modules are now built under npm/dist
• ./npm/package-lock.json has been added (deps versions updated)
• ./npm/package.json has been refactored

How janus.js packages will be published

We have added a prerelease script to ./npm/package.json that generates an ES version of janus.js through rollup under ./npm/dist/janus.es.js and copy janus.js under ./npm/src/janus.js

So basically the publishing cycle will be the following (remove --dry-run)

# update version in ./npm/package.json
# npm run prerelease
# npm publish --dry-run

The package published on npm will have the following contents:

README.md        ---> the README that will be shown on npm
dist/janus.es.js ---> the "main" of the package.json as generated by rollup
janus.d.ts       ---> the "types" of the package.json
package.json    
src/janus.js     ---> copied from "html/janus.js" when doing "npm run prerelease"

Feedbacks are very welcome, in particular about the project structure we defined and how we think to manage the release cycle.

[lminiero]

Merging :+1: