Add access to interpolation fraction for fixed timestep interpolation

Addresses #30068

This is a prerequisite for allowing proper support for fixed timestep interpolation, exposing the interpolation fraction to the engine, modules and gdscript.

The interpolation fraction is the fraction through the current physics tick at the time of the current frame.

main/main_timer_sync.h

@@ -36,6 +36,7 @@
 struct MainFrameTime {
 	float idle_step; // time to advance idles for (argument to process())
 	int physics_steps; // number of times to iterate the physics engine
+	float interpolation_fraction; // fraction through the current physics tick
 
 	void clamp_idle(float min_idle_step, float max_idle_step);
 };