Add OS::is_process_running function.

Adds the is_process_running function to the native OS class and exposes it to script.

This is implemented on Windows and Unix platforms. A stub is provided for other platforms that do not support this function.

Documentation is updated to reflect new API function.

platform/uwp/os_uwp.cpp

@@ -647,6 +647,10 @@ Error OS_UWP::kill(const ProcessID &p_pid) {
 	return FAILED;
 }
 
+bool OS_UWP::is_process_running(const ProcessID &p_pid) const {
+	return false;
+}
+
 Error OS_UWP::set_cwd(const String &p_cwd) {
 	return FAILED;
 }