[ASR] Expose access to "special folder" paths #869

Currently autosplitters can only get the path of the executable they're attached to. Sometimes the file you want to read is not relative to this - e.g. if it's in my documents or under %appdata%. There should be some way to get at these "special folders".

Rather than dealing with string substitutions, I think dotnet has a relatively clean interface we could borrow:

string path = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);

Now dotnet tries to map the same enum values across platforms - e.g. my documents becomes XDG_DOCUMENTS_DIR on linux (source). Since we're trying to replicate whatever logic the game's doing, I think it's better for them to be explicitly different, any logic we implemented might not work the same as the game's.

My proposed API would be something like:

enum SpecialFolder : uint32_t {
    WIN_DOCUMENTS,
    WIN_APPDATA,
    WIN_LOCAL_APPDATA,
    // ...
    XDG_DOCUMENTS,
    // ...
};
bool runtime_get_special_folder(SpecialFolder folder, char8_t* buf, uintptr_t* len);

This would have the same semantics as process_get_path - so trying to get WIN_DOCUMENTS on a linux host would return false/0 length.

As an additional complication, when running a windows game under wine, you'll want to be able to get the special folders in the emulated windows filesystem, rather than on the host linux system. From what I can tell, the best process for finding these is:

• Pick the process you want to get the path for - different processes may have different wine prefixes, and thus different filesystems
• Search /proc/{pid}/environ for WINEPREFIX. If it doesn't exist, assume ~/.wine.
• Read though $WINEPREFIX/user.reg (which stores HKEY_CURRENT_USER), and look for the shell folders. There are two sections, I'm not entirely sure which one takes priority, but I don't imagine many people are manually editing these files anyway, let alone only editing one.

Since this needs to be based on a specific process, it would need another export:

bool runtime_get_wine_special_folder(AttachedProcess process, SpecialFolder folder, char8_t* buf, uintptr_t* len);