Add spa-loading-template.html to content config

[nikolasdas]

Describe the feature

Hey everybody,

This is a very small thing, but I noticed that the optional spa-loading-template.html was not included in the tailwind content config. It's easy to add this yourself of course, but I thought it might be a good idea to include it in the default config, because otherwise a class only used in the loading template would not be included in the bundle :)

Additional information

• [x] Would you be willing to help implement this feature?
• [ ] Could this feature be implemented as a module?