Document that NodeJS >=10 is supported and run CI for multiple NodeJS versions

[Minnozz]

Fixes #5189

[bobzhang]

The test failure seems to be that in recent node versions, unhandled promises rejections is considered a failure.
It can be ignored temporarily.
I wonder if you can give a meaningful name
currently: ci/circleci:build-test-1 -> ci/circleci:node-v10

Edit: I am also wondering what's the motivation here? In general, rescript is not sensitive to node versions, node is just used as a thin wrapper

[Minnozz]

I've given the jobs meaningful names.
My motivation was that when I was working on #5181, I accidentally broke rescript_format.js on NodeJS v10 because I imported fs/promises which does not work on older NodeJS versions.
Even though the JavaScript wrapper is only a thin layer, it would be bad if we accidentally break the whole of the ReScript toolchain for a subset of users by making such a mistake as I did.
Of course, which NodeJS versions we want to support exactlyis a decision for the team, but this way we can at least easily verify that it does work on those versions.

[bobzhang]

, it would be bad if we accidentally break the whole of the ReScript toolchain

I understand the risk, but it is really low in reality if we only test the CI on the oldest node we support. The thing is that doing CI for all versions brings lots of noise

[Minnozz]

The last commit in this PR changes the CI to two test environments: linux-node-v10 and macos-node-v10.

[peterpme]

👋 since node v12 and v14 are in maintenance mode, should node v12 become the new lowest supported version?

[cristianoc]

Now stale.