Enable ES Module usage

[benschwarz]

As noted in #80, es module imports are currently broken on the 2.1.0 release.

According to the node docs, the package.json module field is ignored. Instead, an ES module can be signified using "type": "module".

In this PR:

• Bump to 2.1.1
• Add "type": "module"

[benschwarz]

@jhurliman It looks like we'll need a similar change to just-performance too. Did you want me to go ahead and update that as well?

[nigelis]

@benschwarz
Adding "type": "module" makes this module not importable by other commonjs modules, as I tested locally.

The change to remove "type": "module" is introduced in #79 .
I think the real problem is that the module is not built properly to support esm and cjs at the same time.