Basic convert to esm with only default exports

[TrySound]

Ref #2012

I decided to make at first basic conversion with only default exports. Also I added esm loader.

[TrySound]

before

cytoscape.js 815.1K
cytoscape.min.js 325.2K
cytoscape.umd.js 795.9K

after

cytoscape.js 888.0K
cytoscape.min.js 327.0K
cytoscape.umd.js 868.8K

seems like currently size increased with 2kb, but it should be fixed with rollup's scope hoisting

[TrySound]

Benchmarks has a lot of lint errors so I'd like to convert them later.

[TrySound]

@maxkfranz Thoughts?

[maxkfranz]

It looks OK from what I saw before. I just haven't had time to test and merge...

I'll try to set aside some time tomorrow to try it out (and possibly merge).

[maxkfranz]

Some questions re. next steps:

(1) Should we be specifying 'module' in package.json? http://2ality.com/2017/07/npm-packages-via-babel.html

(2) Does Rollup support everything we're currently doing with Webpack in the webpack.config.js and the package.json?

[maxkfranz]

Re (2), we need:

(a) Watch support : --watch via CLI: https://rollupjs.org/guide/en#command-line-flags

(b) Inline source maps toggled on/off via config file: https://rollupjs.org/guide/en#output-sourcemap-m-sourcemap

(c) A way to minify with Uglify and toggle it on/off with an env var in the config file: Not sure if this is possible. We could just pipe the output to uglify, but then Windows users can no longer build. For myself, that's not a problem but it might discourage Windows users from contributing to the lib.

(d) Babel support: https://rollupjs.org/guide/en#babel

(e) Making a UMD build that supports external refs to dependencies. Not sure if this is supported.

(f) Bundling dependencies from npm that aren't ES modules: It looks like a plugin does this https://rollupjs.org/guide/en#npm-packages

(g) Something analogous to EnvironmentPlugin from Webpack : Not sure if this exists

[TrySound]

1. Yes. module field will make sense when we will land esm bundle.
2. Yes. Rollup support everything we need.
   a. rollup-watch
   b. yes
   c. There is cross-env package which works for all shells.
   d. rollup-plugin-babel
   e. yes
   f. rollup-plugin-node-resolve and rollup-plugin-commonjs
   g. rollup-plugin-replace