Rework variants to reduce bundle size

[MattiasBuelens]

This removes some variants and build artifacts to reduce the total download size of the npm package.

• Remove all generated source maps.
• Remove the ES2018 variant, which is now combined into the ES6 variant. This means that ReadableStream.prototype[Symbol.asyncIterator] won't correctly inherit from %AsyncIteratorPrototype%, but nobody cares about that. 😛
• Remove unminified polyfill variants. All build artifacts for the polyfill variants are now always minified, while the ponyfill variants are always unminified.
• Switch to subpath exports for the different variants.
• Remove down-leveling for TypeScript 3.5 or lower. We need at least TypeScript 4.7 for it to support the package entry points anyway.

Ported from #83 and #107.

[MattiasBuelens]

Package size with this PR:

$ npm pack
npm notice 
npm notice package: [email protected]
npm notice === Tarball Contents ===
npm notice 1.1kB  LICENSE
npm notice 6.4kB  README.md
npm notice 70.6kB dist/polyfill.es5.js
npm notice 64.7kB dist/polyfill.js
npm notice 70.7kB dist/ponyfill.es5.js
npm notice 70.5kB dist/ponyfill.es5.mjs
npm notice 64.8kB dist/ponyfill.js
npm notice 64.2kB dist/ponyfill.mjs
npm notice 3.2kB  package.json
npm notice 1.2kB  types/polyfill.d.ts
npm notice 33.8kB types/ponyfill.d.ts
npm notice 329B   types/tsdoc-metadata.json
npm notice === Tarball Details ===
npm notice name:          web-streams-polyfill
npm notice version:       3.3.2                                   
npm notice filename:      web-streams-polyfill-3.3.2.tgz
npm notice package size:  98.4 kB
npm notice unpacked size: 451.5 kB
npm notice shasum:        4665d4e93355835fbc6f191280b0e118bc0b86a2
npm notice integrity:     sha512-qlYtN3YEOFidz[...]59brCzcbDKVyw==
npm notice total files:   12
npm notice