Manually version browsers

[dgp1130]

@io_bazel_rules_webtesting doesn't appear to be maintained anymore and hasn't see a release in over 2 years. This means the version of downloaded browsers is falling out of date.

It wouldn't be that big a deal, except that I've been using <template shadowroot="open"> because that is what's supported in the version of Chrome used for testing. However, the spec was changed to use <template shadowrootmode="open"> and Chrome M119 is slated to remove support for <template shadowroot="open">. This is currently scheduled for Nov 7th, per https://chromiumdash.appspot.com/schedule.

As a result, I need to update my code to use shadowrootmode, which means I need a recent-enough browser version which supports that. I'm wondering if I can fork or build a simpler solution good enough for @rules_prerender. This is definitely blocking 1.0.0 because I wouldn't want to ship that until I can use shadowrootmode correctly.

I have a prototype in ref/browser-update forked from an implementation in the Angular Dev Infra repository. This was sufficient to download recent versions of Chrome / Firefox and get a passing test with shadowrootmode on Chrome.

Some more things I'll need to figure out:

• Firefox - Doesn't seem to be starting correctly in WSL right now. I would love to do better cross-browser testing, but Firefox doesn't support declarative shadow DOM right now, which is a big part of @rules_prerender. It might be fine since I have the polyfill, and could serve as a test case for that. But we may want to think about how much value this is actually giving and whether or not its worth it.
• Debugging - I'm struggling to start headful (?) Chrome to debug a test like I've done in the past. Might take some trial and error to figure that out.
• Test everything - New Chrome browser should be enabled everywhere in the repository.
• Updating browsers - There's a script in Angular Dev Infra for updating browser version automatically which would be useful to fork as well.
• Refactor - I kept the existing directory structure and formatting conventions which definitely clashes with @rules_prerender. I'll want to refactor things to make it more maintainable and aligned with the rest of the codebase.
• Migrate to shadowrootmode - Once everything is stable, I should be able to migrate all usages of shadowroot to shadowrootmode.
• Break @io_bazel_rules_webtesting dependency - If possible, I should also look into breaking the dependency on @io_bazel_rules_webtesting entirely. Unfortunately the Angular implementation does still rely on a few exported symbols from that ruleset, even if it versions its own browsers. It would be great to drop that dependency altogether.