support import from node versions < 12.7

[reaganr]

Node versions < 12.7 can't import non-.js files (such as .json) that aren't explicitly exposed from the package.json using "main" or similar explicit export properties.

[kemitchell]

I'm pretty sure Node.js versions from well before the io.js split supported index.json requires without package.json meta. If this changed---or changed and changed back---more recently, do you happen to know in which versions?

I'm not against spec'ing main in package.json.