C#/netcore: Add base desktop game export implementation

This base implementation is still very barebones but it defines the path
for how exporting will work (at least when embedding the .NET runtime).

Many manual steps are still needed, which should be automatized in the
future. For example, in addition to the API assemblies, now you also
need to copy the GodotPlugins assembly to each game project.

modules/mono/utils/path_utils.cpp

@@ -51,6 +51,37 @@
 
 namespace path {
 
+String find_executable(const String &p_name) {
+#ifdef WINDOWS_ENABLED
+	Vector<String> exts = OS::get_singleton()->get_environment("PATHEXT").split(ENV_PATH_SEP, false);
+#endif
+	Vector<String> env_path = OS::get_singleton()->get_environment("PATH").split(ENV_PATH_SEP, false);
+
+	if (env_path.is_empty()) {
+		return String();
+	}
+
+	for (int i = 0; i < env_path.size(); i++) {
+		String p = path::join(env_path[i], p_name);
+
+#ifdef WINDOWS_ENABLED
+		for (int j = 0; j < exts.size(); j++) {
+			String p2 = p + exts[j].to_lower(); // lowercase to reduce risk of case mismatch warning
+
+			if (FileAccess::exists(p2)) {
+				return p2;
+			}
+		}
+#else
+		if (FileAccess::exists(p)) {
+			return p;
+		}
+#endif
+	}
+
+	return String();
+}
+
 String cwd() {
 #ifdef WINDOWS_ENABLED
 	const DWORD expected_size = ::GetCurrentDirectoryW(0, nullptr);