make esm treeshake friendly and add test

[loynoir]

This PR

1. makes js-yaml esm treeshake friendly
2. new test added
3. NOT update dist, to get it easier to review
4. 2+3, leads to CI failure

This PR + updated dist

1. can be found in branch https://github.com/loynoir/js-yaml/tree/feat-esm-treeshake-friendly-with-dist
2. tests pass on node10, node12, node 14, node16, node17
3. fixed https://github.com/nodeca/js-yaml/issues/631

[alexander-riss]

@puzrin any opinion on this? it looks like this might help downstream projects cut down on bundle size via tree-shaking

[seravifer]

Any updates?