Warn during test when Node 12+ isn't used

[WilcoFiers]

PR #3538, axe-core is back to using Node v4 as its minimum. That's true enough when it comes to importing and using axe, but Node 12+ is required for building / testing. We should add an explicit warning of sorts to call out when an older version of Node is used to build / test axe.

I am very open to how to do this. Easiest might be to add some little script that returns exit code 1 in the pretest and prebuild scripts in package.json.

[pandu-supriyono]

Hi, I have the time and motivation to look into this.

Do I understand correctly that we want to produce a fatal error with exit code 1 if the minimum Node version isn't found instead of a warning message that exits with exit code 0?

Edit: As a side note, it would also be useful to include this requirement in the developer guide.

[straker]

Thanks for taking this on @pandu-supriyono. I believe having the build script or test script exit with a clear message and exit code 1 if Node version is < 12 would be fine. Adding a note in the developer guide on minimum Node version would also be very helpful.