Enhancement: Load mods from XDG_DATA_HOME

[OPNA2608]

Please check for duplicates or similar issues before creating this issue.

I don't know how feasible this is since polymod seems to only operate on one modRoot, but I'll try to explain the problem I have & ideal solution.

What is your suggestion, and why should it be implemented?

What?

I'd like to suggest checking the regular mods directory first (to accommodate most OS' and system-installed mods), then entering an #if linux-gated block of code where the XDG Base Directory Specifications are referenced: The $XDG_DATA_HOME environment variable is resolved (fallback to $HOME/.local if unset) and some unique path in $XDG_DATA_HOME/share/ is checked via Polymod for any user-installed mods.

Why? (What on earth does any of that even mean?)

Currently the game ships/builds with a hardcoded directory location for mods. This is perfectly adequate for Windows and macOS desktops where users expect to go to a download page to install the game. People who manually build the game from source would also hardly have a problem with this approach.
https://github.com/ninjamuffin99/Funkin/blob/46d19f1170a1faa197d2c0079d3bde6e1cfed531/source/TitleState.hx#L49-L51

On Linux however, people usually prefer using their package manager of choice to install applications. I'm working on such a package and I noticed a problem with this mods approach: When the application & its data is installed, it's usually in a place that requires system administrator permissions to modify (any Linux version comparable to Ubuntu) or in some cases is completely unavailable for post-installation modifications for security/purity reasons, even as an admin (my Linux version). As such: adding mods is impossible to do without copying the game's files to another user-accessible directory, fixing file/directory ownership & permissions etcetc.

The Linux world's way of handling this is to use the XDG Base Directory Specifications. $XDG_DATA_HOME/share (usually resolves to /home/<currentuser>/.local/share) is a directory where applications are supposed to write any user-specific data they want, but it's also the de-facto place for users to supply their own data through, e.g. user-installed fonts in $XDG_DATA_HOME/share/fonts. The game already uses this specification somewhere(!) to store highscores and some other stuff locally ($XDG_DATA_HOME/share/ninjamuffin99/Funkin/ninjamuffin99/funkin.sol on my system) so maybe that directory could house an additional mods directory as well?