'Open Mods Folder' button doesn't respect custom mods folder setting

[williambl]

The button opens the mods folder within the current game directory, but that's not guaranteed to be the actual mods folder. - https://github.com/TerraformersMC/ModMenu/blob/438d16e69ad40962a12ead6bdb5acc1a42857bcb/src/main/java/com/terraformersmc/modmenu/gui/ModsScreen.java#L236

fabric-loader will set the mods folder location according to the fabric.modsFolder system property - https://github.com/FabricMC/fabric-loader/blob/8fc5065227f72b5a00ae4c0376398120776926a9/src/main/java/net/fabricmc/loader/impl/util/SystemProperties.java#L47

Unfortunately fabric-loader does not expose a method in its API to get the mods folder, but modmenu could also read the system property.