Add a system property to disable unsupported mod warnings (#434)

The system property is `loader.ignore_unsupported_mods` - set this to true with `-Dloader.ignore_unsupported_mods=true` to ignore all unsupported mods.

This is a straightforward solution to #433.

src/main/java/org/quiltmc/loader/impl/plugin/QuiltPluginManagerImpl.java

@@ -871,6 +871,11 @@ private void populateModsGuiTab(ModSolveResultImpl result) {
 			}
 		}
 
+		if (SystemProperties.getBoolean(SystemProperties.IGNORE_UNSUPPORTED_MODS, false)) {
+			/* skip unsupported mod checking */
+			return;
+		}
+
 		Map<UnsupportedType, SortedMap<String, UnsupportedModDetails>> filesList = new HashMap<>();
 
 		for (PathLoadState loadState : modPaths.values()) {

src/main/java/org/quiltmc/loader/impl/util/SystemProperties.java

@@ -98,6 +98,8 @@ private SystemProperties() {}
 	public static final String ALWAYS_DEFER_FILESYSTEM_OPERATIONS = "loader.workaround.defer_all_filesystem_operations";
 	public static final String DISABLE_QUILT_CLASS_PATH_CUSTOM_TABLE = "loader.quilt_class_path.disable_custom_table";
 	public static final String DISABLE_BUILTIN_MIXIN_EXTRAS = "loader.disable_builtin_mixin_extras";
+	/** whether the loader should display unsupported mods with the GUI or ignore and continue starting up the game. */
+	public static final String IGNORE_UNSUPPORTED_MODS = "loader.ignore_unsupported_mods";
 
 	/** Disables loader from registering its {@link URLStreamHandlerFactory} with
 	 * {@link URL#setURLStreamHandlerFactory(URLStreamHandlerFactory)}. This */