External review of the chosen paths to standardized directories on each platform

[soc]

Many of the paths to standardized directories are thoroughly documented by each operating system vendor, but it would still be helpful for someone not me to go through them and give things a second review.

There are also cases where some directories just don't exist or have a very different behavior:

For instance executables_dir: On Linux, one can expect that binaries placed in this folder also end up on the $PATH, but no such folder exists on macOS and Windows.

It would be helpful to go through the ones that return Option<PathBuf> and make sure these provide the best possible experience given the circumstances.

• executable_dir: Resolved, only provided on Linux.
• Cache and data directories on Windows, see Consider using config and data subdirectories for {FOLDERID_RoamingAppData} on Windows #9.
• Review of XDG user directory values on Linux – what should happen if no values are present? Fallback or None?

[soc]

XDG user dirs return None instead of inventing some fallback, this is consistent with directories-jvm.