Lazy Images: Improve JS handling

[kraftbj]

Originally reported in #17489 (comment) via @ockham regarding the Lazy Image package JS.

• We're only bundling code here (through Webpack), not transpiling it (through Babel). This means that statements like const end up in built files, as detected by the ES5 validation script.
• We're dropping our built files (*.min.js) into src/, rather than a separate location.
• We're adding built files to GH.
• We're including the Interaction Observer polyfill verbatim.

In order to fix these issue, [he] suggests

• using calypso-build (which takes care of transpilation, and by default drops files into a separate build/ directory, thus resolving items 1. and 2.)
• deleting built files from GH, and adding the build/ subdirectory to .gitignore in order to prevent that from happening in the future
• adding the Interaction Observer npm to package.json (rather than copying it verbatim)
• Revert Build: Remove lazy-images ES5 validation #17561 to re-enable validation.

[kraftbj]

One thing we'd need to figure out is since this is a Composer package, it is likely expected to be consumed by other plugins without the need to incorporate it into a JS build step? (the original push to get this into a package was for a non-Jetpack consumer: #16657 )

In either case, removing it would break BC for consumers, so that needs to be handled in some way.

[jeherve]

We're dropping our built files (*.min.js) into src/, rather than a separate location.
We're adding built files to GH.

I think this is fine. We'd want the package to be ready to use by anyone adding the package to their composer.json file.

[jeherve]

using calypso-build

We should be able to update webpack.config.packages.js to start transpiling things, that should solve our issue here and ensure that the Lazy Images lib. can be used in old browsers again.