Restore the BOM-skipping logic to the Lua script reader

And add BOM to one of the test files to verify it works.

automation/include/aegisub/unicode.moon

@@ -1,4 +1,4 @@
--- Copyright (c) 2007, Niels Martin Hansen, Rodrigo Braz Monteiro
+-- Copyright (c) 2007, Niels Martin Hansen, Rodrigo Braz Monteiro
 -- All rights reserved.
 --
 -- Redistribution and use in source and binary forms, with or without

libaegisub/lua/script_reader.cpp

@@ -38,6 +38,12 @@ namespace agi { namespace lua {
 		auto buff = file.read();
 		size_t size = static_cast<size_t>(file.size());
 
+		// Discard the BOM if present
+		if (size >= 3 && buff[0] == -17 && buff[1] == -69 && buff[2] == -65) {
+			buff += 3;
+			size -= 3;
+		}
+
 		if (!agi::fs::HasExtension(filename, "moon"))
 			return luaL_loadbuffer(L, buff, size, filename.string().c_str()) == 0;