Refactor TypeScript definition to use CJS compatible export

[BendingBender]

Fixes #31.

Waiting for tsdjs/tsd#20.

[fregante]

I tried all of these combinations and all seem to be working correctly (approved by TS or showing autocomplete in VSCode.

// TS file
import mem from 'mem';
import mem = require('mem');

// JS file
import mem from 'mem';
const mem = require('mem')

So I approve this style, except the two comments below:

[fregante]

Your reasoning makes sense, and also the intent to drop the default export in future releases to avoid breaking existing setups without esModuleInterop 👍

[BendingBender]

#29 probably should be merged before this one.

[sindresorhus]

#29 probably should be merged before this one.

✅

[sindresorhus]

Can you add a TODO comment to https://github.com/sindresorhus/mem/blob/master/index.js#L79 too?