RxJS 7 Modules/Types/Package Structure Epic

There's been a smattering of issues around our package structure, locating types, pulling in the proper versions, etc.

• Ensure ESM users are getting ES2015/ESM build (must be 8.x)
• Ensure CJS users are getting CJS build
• Ensure those that want ES5 (whomever they are?) are getting ES5 build (not supported in 8.x)
• Ensure it works with the latest Angular-CLI
• Ensure it works with the latest Create React App
• Ensure it works with Vite/Vue builds
• Ensure it works generally with Rollup/Webpack
• (Somehow) ensure it works with Stackblitz
• Make sure people don't end up with two versions of RxJS in mixed environments, if possible. (e.g. import { Observable as Observable1 } from 'rxjs'; require('rxjs').Observable === Observable1) Again, I'm not even sure this is possible
• Figure out some sort of way to test this in CI so it doesn't happen again.
• Figure out if there are any Electron issues

Related Issues:

• #6321 ESM version is never used
• #6786 Make sure whatever we output from Nx/whatever for 8.x meets the above requirements.

Start by reading related issues #6321 and #6786, then inspect the RxJS 8.x package and build configuration. Define reproducible checks for ESM, CJS, ES5, Angular CLI, Create React App, Vite, Rollup, Webpack, StackBlitz, Electron, and mixed-version usage; done requires those checks and CI coverage.