Enforce js file extension in imports

[zepumph]

From phetsims/chipper#1498. There is a plugin that we can add a lint rule for:

Two options:
https://github.com/import-js/eslint-plugin-import/blob/67cc79841fc823ad4af2532af2dc6704e4b3b03a/docs/rules/extensions.md
https://github.com/solana-labs/eslint-plugin-require-extensions

@samreid, what do you think?

[samreid]

Is the existing bad-text lint rule insufficient? Do we need something with --fix?

[zepumph]

I experimented with that, but it is a pretty naiive test, as it doesn't support a case like import winston from 'winston';.

I updated it and moved it to the base bad-text rule, but we may still want to swap things out to use a plugin if we find another corner case or two in the future.

[chrisklus]

There's a few website imports getting caught by this, but it's unclear to me why all imports that don't have a suffix are not breaking, see https://github.com/phetsims/website-meteor/issues/813. Any ideas?

[zepumph]

Well that isn't ideal! I think the best thing for now is to turn off this error until we can implement the plugin directly as an actual solution.