Extract functions for service worker usage, and add initial MSC3916 playwright test (when supported) (#12414)

* Send user credentials to service worker for MSC3916 authentication

* appease linter

* Add initial test

The test fails, seemingly because the service worker isn't being installed or because the network mock can't reach that far.

* Remove unsafe access token code

* Split out base IDB operations to avoid importing `document` in serviceworkers

* Use safe crypto access for service workers

* Fix tests/unsafe access

* Remove backwards compatibility layer & appease linter

* Add docs

* Fix tests

* Appease the linter

* Iterate tests

* Factor out pickle key handling for service workers

* Enable everything we can about service workers

* Appease the linter

* Add docs

* Rename win32 image to linux in hopes of it just working

* Use actual image

* Apply suggestions from code review

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

* Improve documentation

* Document `??` not working

* Try to appease the tests

* Add some notes

---------

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

playwright/element-web-test.ts

@@ -33,6 +33,10 @@ import { Bot, CreateBotOpts } from "./pages/bot";
 import { ProxyInstance, SlidingSyncProxy } from "./plugins/sliding-sync-proxy";
 import { Webserver } from "./plugins/webserver";
 
+// Enable experimental service worker support
+// See https://playwright.dev/docs/service-workers-experimental#how-to-enable
+process.env["PW_EXPERIMENTAL_SERVICE_WORKER_NETWORK_EVENTS"] = "1";
+
 const CONFIG_JSON: Partial<IConfigOptions> = {
     // This is deliberately quite a minimal config.json, so that we can test that the default settings
     // actually work.