fs-api: extend host API to allow retrieval of a resource file path

pcsx2/Host.h

@@ -41,4 +41,7 @@ namespace Host
 
 	/// Reads a resource file file from the resources directory as a string.
 	std::optional<std::string> ReadResourceFileToString(const char* filename);
+
+	/// Returns the full filepath to a resource file, if it exists.
+	std::optional<std::string> getResourceFilePath(const char* filename);
 } // namespace Host

pcsx2/gui/AppHost.cpp

@@ -66,3 +66,14 @@ std::optional<std::string> Host::ReadResourceFileToString(const char* filename)
 
 	return ret;
 }
+
+std::optional<std::string> Host::getResourceFilePath(const char* filename) 
+{
+	const std::string full_filename(Path::CombineStdString(EmuFolders::Resources, filename));
+	if (!FileSystem::FileExists(full_filename.c_str()))
+	{
+		Console.Error("Resource file does not exist '%s'", filename);
+		return std::nullopt;
+	}
+	return full_filename;
+}