Browser support tiers and on-demand E2E browser matrix testing

[doodlewind]

For speeding up CI, we only run E2E tests against Chromium by default, which is our Tier 0 supported browser.

But the situation is that we have 1/3 of seed users using Firefox. And with Tauri, we need to support WebKit on macOS (WebView 2 on Windows is Chromium-based, good news for us). So Firefox and Safari should be our Tier 1 browsers.

The basic support strategy could be:

• For the latest Tier 0 browsers, all master commits should not break its E2E test cases.
• For the latest Tier 1 browsers, we run E2E tests against them daily (fully supported by Playwright), fixing regressions ASAP. Meanwhile, for major refactoring or compatibility-related PRs, this CI process can be manually triggered by commenting on PR.
• For other browsers and older versions of T0/T1 browsers, we accept bug reports and decide the priority case by case.

Tasks here:

1. Support daily scheduled E2E browser support matrix testing, just like what we do in ci: add codeql-analysis.yml #306.
2. Support triggering this test suite manually on GitHub.

cc @tzhangchi

[doodlewind]

Closed by #793 and #796, thanks @fi3ework for the help!