TypeScript support

[egoist]

• Add .js extension to the cache filename if it doesn't have extension
• Figure out how to make types work with the cached dependencies

[hanssens]

If I may add, specifically for TypeScript, there is another issue with compile-time errors when you import a remote module:

import { Foo } from 'https://cdn.whatever.com/module.js';

Without attributing the line above with // @ts-ignore, the TS compiler will spit out Cannot find module... errors.

Similar, this also occurs when using a dynamic import:

async foo(): Promise<void> {
   const module = await import('https://cdn.whatever.com/module.js');
}