feat: delMany

[David Dal Busco]

Feature Suggestion

Would it make sense to also provide a delMany to delete many keyval pairs at once?

import { del, delMany } from 'idb-keyval';

await Promise.all(del('a'), del('b'), del('c')];

await delMany(['a', 'b', 'c']);

[Jake Archibald]

I'm happy for someone to add this (plus tests)