chore: .node-version, .nvmrc and nodenv files

[mightyiam]

Closes #422

[sne11ius]

The fact that this PR lingers for 2 months without any comment makes me question the overall quality of this project to be honest. Seems not to be too actively maintained.
Apart from that: I know I'm not entitled to anything ;)

[paldepind]

Thanks for the PR @mightyiam. I'm wondering how you arrived at v6.10.1 as the NodeJS version?

@sne11ius It's probably fair to say that Snabbdom is not too actively maintained 😉. But it's not unmaintained either and the project has been used by many people for many years. It has been battle tested and is stable. If you have doubts about the "overall quality of the project" that are related to the code itself, let me know. And btw, writing comments, like you did above, is a fine way to increase the chance that I or others will take a look at issues/PRs. So feel free to keep doing that 😄