allow to configure when which schematic gets unlockable

bin/test/data/config/ships.toml

@@ -1,6 +1,6 @@
 [[ship]]
 id = "interceptor"
-available_from_start = true
+unlock_at_station_level = -1
 layout = ["XOX", "OOO", "XOX"]
 default_modules = [{type = "laser_cannon", x = 1, y = 1, rotation = "east"}]
 
@@ -31,7 +31,7 @@ scrap_drop = 2
 
 [[ship]]
 id = "destroyer"
-available_from_start = true
+unlock_at_station_level = -1
 layout = ["XOOX", "OOOO", "XOOX"]
 default_modules = [{type = "laser_cannon", x = 1, y = 1, rotation = "east"}]
 
@@ -62,7 +62,7 @@ scrap_drop = 4
 
 [[ship]]
 id = "salvage_ship"
-available_from_start = true
+unlock_at_station_level = -1
 layout = ["OOO", "OOO"]
 
 [ship.schematic]
@@ -92,7 +92,7 @@ scrap_drop = 2
 
 [[ship]]
 id = "repair_ship"
-available_from_start = false
+unlock_at_station_level = 0
 layout = ["XOX", "OOO", "XOX"]
 
 [ship.schematic]