Map initialization routines

[melody-rs]

Is your feature request related to a problem? Please describe.
It's pretty useful to run event commands when a map is first loaded. Fading Memory does this to add particles and set map backgrounds.

Describe the solution you'd like
Add support for a "map initialization routine": a set of event commands attached to a map that are automatically run when loaded. See #134 for enabling this feature

Describe alternatives you've considered
Currently, the easiest way to do this is to add an autorun event that deletes itself after it finishes processing. This isn't perfect though, the event isn't guaranteed to run immediately and may execute over a couple of frames.

[zimberzimber]

This doesn't necessarily need any script changes if you are to treat it as just another event.
Could use a convention for the event's name the editor would look for.

[melody-rs]

That could work, but it'd have the exact same caveats as the autorun event