In a fact everything in Godot is a Class, every script you make is a Class, so this is actually an easy one, and has multiple solutions you can use.
Either make a reference class called item (unattached script), and make every other item class (script) extend that class. Or make a multipurpose class called item that has extend variables of various stats, and change those variables via editor for each item node you create.
Additionally you can set an item_type in the reference/description of your items, and when using an item from active slot, decide what to do for each item type e.g.:
if active_item.item_type == "potion":
player.health += 5
elif active_item.item_type == "block":
var tile_position = world_to_map(get_global_mouse_position())
tilemap.set_cell(tile_position.x, tile_position.y, active_item.tile)