Load envalid declarations from directories/files

[SimenB]

The way we have our envalid declarations set up at work is to put them by category in different files in a directory, then load the whole directory, merging all declarations, then passing the result through cleanEnv and making it immutable.

After #44 we can drop our immutable proxy wrapper, but we still need our own code to load all en variables. What do you think about adding that capability directly to envalid?

Example:

../frontpage/server/config
├── assets.js
├── cdn.js
├── messages.js
├── nunjucks.js
├── security.js
├── services.js
├── tracking.js
└── trackjs.js

0 directories, 8 files

Each file is e.g security.js

'use strict';

const { bool } = require('envalid');

module.exports = {
    SECURE_COOKIES: bool({
        desc: 'Enable to set `Secure` flag on all cookies',
        default: true,
        devDefault: false,
    }),
};

You can provide multiple directories, or single files of these, and they are merged.
handling of dupes are undefined (in practice last one wins, but you should avoid overlap). That can be defined, though, if you want that.

[af]

That looks like a nice way of organizing your env config. At work we still have things in a single file and it's getting a bit unwieldy. I'm in favor of adding this as long as it's not adding a ton of complexity, and that the fs calls can be worked around for those using envalid from RN.

[SimenB]

We can probably use the same workaround as jest does in order to fool the compiler.

jestjs/jest#4376

Ref #31 & #38

[af]

I'm going to close this as it's not a near-term priority. Definitely happy to review a PR for this if one comes along though!