Improve compatibility between CJS and ESM modules

[pirelenito]

Changes how we export the library to rely in named exports.

This is a breaking change and will be released as a major version bump.

The change to consumers will be that instead of requiring via:

const GitRevisionPlugin = require('git-revision-webpack-plugin')

It should be via:

const { GitRevisionPlugin } = require('git-revision-webpack-plugin')

This is a fix for #66