Add new constant GM_runtime_type that will tell apart GMS2 and GMRT

[DiasFranciscoA]

Is your feature request related to a problem?

Right now there is no fool proof way of knowing if the current runner is GMRT or GMS2.

Describe the solution you'd like

This should be a constant that allows to differentiate between GMRT and current runner (gms2), could be named GM_runtime_type this should return:

• gms2 (for current runner)
• gmrt (for new runner)

Describe alternatives you've considered

No response

Additional context

No response

[DiasFranciscoA]

This will need implementation on both Current Runner and GMRT:

• https://github.com/YoYoGames/GameMaker/pull/4647