Support for automatic selection of runtime

[zero-plusplus]

If runtime is omitted, search for the runtime in the following order.

1. If runtime_v1 or runtime_v2 is set, set it.
2. If there is an AutoHotkey.exe with the same name as the script in the same directory, set its path
3. If the AutoHotkey Launcher is present in the AutoHotkey installation directory, use it to set the optimal AutoHotkey.exe path
4. Set the path to AutoHotkey.exe or v2/AutoHotkey.exe in the AutoHotkey installation directory

If a folder path is specified in runtime, it is treated as the AutoHotkey installation directory.

This allows the automatic runtime selection described above to feature correctly even if you have installed AutoHotkey outside the normal installation directory.