Part 2/4 of #5380

Opening this as a draft was well until I have the rest finished.

This

1. Adds a utility executable resolver function with support for custom values, default names and fallback paths.
2. Adds a utility function for finding an available port on a specific network interface
   • No tests, because I don't think mocking everthing here makes sense, and neither does running this without mocking
3. Adds the trio dependency, as well as pytest-trio and trio-typing
   • trio-typing looks like it's going to get merged into trio soon
   • pytest-trio is used for initializing the runloop in async tests, similar to pytest-asyncio.
     Having implementations for both asyncio and trio seems to cause minor issues with trio's runloop in the tests, and on Windows it triggers a warning when asyncio was run prior to its own runloop. This is related to signal.set_wakeup_fd(). This can be resolved in the future by re-implementing the utils.processoutput based on trio instead of asyncio. The other asyncio stuff is purely concurrency related and not I/O related, so it's irrelevant which runloop we're using. The warning can be ignored for now.
4. Adds the Webbrowser base class with the basic launch implementation
   • I initially had custom process cancellation logic added, when the process doesn't end on SIGTERM on POSIX systems. The default behavior of trio waits for 5 seconds and then sends SIGKILL. My plan was to wait a bit less and log a warning instead of triggering a warning via the warnings system, but I ran into issues with the custom implementation while testing. Some weird internal issues with trio and the stdlib subprocess module. Doesn't matter though.
5. Adds the ChromiumWebbrowser class
   • This includes a list of all known Chromium/Chrome executable names (with Chromium having higher priority)
   • With fallback paths for Windows and macOS (because of PATH env var nonsense)
   • I carefully selected launch parameters for disabling lots of unneeded features and behaviors which we don't need or which are bad for our purposes. This is all annotated and linked. There's probably more that could be added.
   • Headless mode is currently not added, but this can be done with a simple class argument. I can't get a valid client-integrity token on Twitch with headless mode, regardless of what others are saying, so I don't see the point right now.
   • Chromium-based browsers I tested (CLI arguments for custom paths are not included in this PR):
     • Chromium (Arch): worked
     • Google Chrome (Windows): worked
     • Brave (Windows): worked
     • Vivaldi (Windows): didn't work, despite a successful CDP session, didn't check further
     • Opera (Windows): didn't work, didn't check further