Synchronize tsconfig targets in the monorepo

[kckst8]

multiple different targets are being used. All should be "es2020". Ensure that everything builds and runs after the change

[aruniverse]

Take a look at:

• https://kangax.github.io/compat-table/es2016plus/ & sort by Engine types
• What TypeScript configuration produces output closest to Node.js 16 capabilities?
  • I would keep the module as ES2020, and would not want to set "type": "module" in the package.json yet, I don't want to start importing from *.js files yet :/

In the tsconfig for itwin/viewer-react, itwin/web-viewer-react, itwin/desktop-viewer-react, they extend the one found in core, so you will need to manually add an entry for "target": "ES2020" to override whats in the base.

Please test the web-viewer-test app in the repo, I think last summer when i tried to update to es2020, webpack4 didnt work with it.