Fix errors under 5.x and allow selecting warps from a dropdown (#5)

- Loading would previously fail if there was both no data in mod_storage "warps" and no warps.txt
- A wrong variable used while adding the warp queue was causing crashes when attempting to warp
- Don't even attempt to teleport to a warp spot that doesn't exit
- Allow selecting warps from a dropdown for ease of use, but still allow text input as warps might be created later than the warp crystals are created (negotiable, invalid input could be dropped and only the combobox allowed if others object).

.luacheckrc

@@ -0,0 +1,16 @@
+read_globals = {
+	"DIR_DELIM",
+	"core",
+	"dump",
+	"vector", "nodeupdate",
+	"VoxelManip", "VoxelArea",
+	"PseudoRandom", "ItemStack",
+	"AreaStore",
+	"default",
+	table = { fields = { "copy", "getn" } }
+}
+
+globals = {
+	"minetest"
+}
+