Replace/inline dead `map-age-cleaner` package

[fregante]

The package is outdated and it has an old p-defer dependency:

• Release as ES Module SamVerschueren/map-age-cleaner#9
• https://github.com/SamVerschueren/map-age-cleaner/blob/0f9f8631510815dd7f567372461fa923c25f2c46/package.json#L38

Related:

• Update dependencies #91

[sindresorhus]

I think we can just inline it. It's a simple package.

[fregante]

Copy tests and all?

• map-age-cleaner/index.js
• map-age-cleaner/test.js
• map-age-cleaner/license

?

[sindresorhus]

Yeah, we can just vendor it for now.