Typescriptify & use service worker for MSC3916 authentication (#27326)

* Typescriptify & use service worker for MSC3916 authentication

* appease the linter

* appease jest

* appease linter

* Get the access token directly

* Add a bit of jitter

* Improve legibility, use factored-out functions for pickling

* Add docs

* Appease the linter

* Document risks of postMessage

* Split service worker post message handling out to function

* Move registration to async function

* Use more early returns

* Thanks(?), WebStorm

* Handle case of no access token for /versions

* Appease linter

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Remove spurious try/catch

* Factor out fetch config stuff

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Finish applying code review suggestions

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Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

res/sw.js

@@ -1 +0,0 @@
-self.addEventListener("fetch", () => {});