Use terser, enhance rollup

[kum-deepak]

Ref #1595

This has package.json change. So, please run npm i. Also, I was not sure if I should checkin the package lock file, so, committed that as a separate commit.

This PR starts using terser, but does some more.

I have created a rollup config which does the following:

• UMD and corresponding minified file in dist/es6
• Flattened ESM and corresponding minified file in dist/esf (useful for importing as modules)
• Split ESM in dist/esm
• Minified ESM split modules in dist/esm-min

Currently it is not integrated with grunt. To test it, run the following:

$ rm -rf dist/*; npx rollup --config

I have included b0b66a7 which is part of #1601. I wanted to verify that with this change, the ESM version does not have import * from 'd3'.

Please see the output. For me also it was first such attempt.

It is clear from the output what rollup module splitting gives. We need to decide which of these six different output formats we need to include in our distribution.

The ESM output also shows that while our code imports everything from d3, the rollup output uses named imports.

[kum-deepak]

Closed in favor of #1603, simpler PR with fewer more relevant commit.