Allow configuring primary line X/Ys separately

[JohnGabrielUK]

This is an implementation of the feature outlined in this proposal, which at time of writing has yet to be approved.

• Bugsquad edit, closes: Allow configuring the primary line x/y components independently for the 2d editor grid snap godot-proposals#7588

[KoBeWi]

The implementation looks fine, but there is a concern of compatibility with old values. If someone set primary step values before, they will be lost after this PR.

This code would handle it:

#ifndef DISABLE_DEPRECATED
	if (state.has("primary_grid_steps")) {
		primary_grid_step_x = state["primary_grid_steps"];
		primary_grid_step_y = state["primary_grid_steps"];
	}
#endif

(not sure if DISABLE_DEPRECATED really applies here; maybe a comment would be sufficient. This value won't be needed in the future)

[akien-mga]

Thanks!