refactor: update runtime directory structure

[R1kaB3rN]

• Updates the file structure of $HOME/.local/share/umu to store container runtimes in subdirectories organized by codename. This obsoletes the files mtree.txt.gz, pressure-vessel, README.md, run, run-in-sniper, sniper_platform_*, steampipe, toolmanifest.vdf, umu, umu-shim, var, and VERSIONS.txt in the top-level of $HOME/.local/share/umu. This change is necessary in preparation for a potential change in the Steam Linux Runtime base platform, and to support switching to different runtimes. Manual intervention will be required by the user to remove the files.
• Adds support for explicitly changing to a specific container runtime.
• Adds support for automatically assigning the correct container runtime for PROTONPATH.
• Adds support for using obsolete Protons down to v5.13. Helpful for clients (e.g., Heroic Games Launcher) which allow its users to use obsolete Protons for their games.

[R1kaB3rN]

Merging for now. Will write the tests and try to add support for the configuration file usage later.