Draft: bin: Add steam-runtime-launch-options script

For steamrt/tasks#77.

This is based on, and mostly replaces, p-v-test-ui. Instead of being part of the pressure-vessel layer, it operates by being added to Launch Options on a per-game basis, which means you only need to change each game's persistent configuration in the Steam UI once, and it can switch between LD_LIBRARY_PATH and container runtimes.

By default, it has no practical effect on the game, and just passes through its arguments as-is. Any changes made to the launch options only affect the current run, and are not "sticky" for subsequent runs. This should make it easier to test new/proposed runtime versions: we can run a game in the proposed runtime, and if it crashes or fails, immediately re-run it in the beta or stable runtime for comparison, with fewer clicks than changing the game's Properties in Steam.

I'm still working on this, it's not ready for merge yet. I'll probably merge it unreviewed since it's only a debug tool and is off the critical path.

/cc @denittis