ES6 promises like using undefined as their first argument instead of null. Both should behave the same behavior

ES6 promises like using undefined as their first argument instead of null. Both should behave the same

We are ok with tests throwing on undefined access, so use the not null assertion heavily here

Fixes #62892

`guessMimeTypes` can be changed by extensions. This is not desirable for webviews. Just hardcode a list of common mimes to use for webview content

Fixes #64777

Start pinning some tests with strict null checks to catch potential regressions

Part of #60565

ref #64754

SelectBox: constructor, interface refactor.  Addresses: #58922